Four years after 2016 election, it hasn't changed that the majority of non-college-degree white people support Trump's reelection, while the majority of college-degree white people don't. Why does having a college degree or not make a difference among white Americans? High school also teach similar topics as colleges, such as science, social science, .... I think most of them, with or without college degree, choose a president in the same reasonably understandable way: what policies benefit their daily lives the most. How does a college degree separate white Americans in what policies benefit them and what not? Thanks.
The experience of getting a degree exposes you to a wider circle of people.
I'm white, male, and went to a school near where I grew up. The school and church had an intertwined population, and my social circle from the neighbourhood, school and church, all overlapped. There were a lot of people just like me, all white, all middle-lower class. Your friends-friends were just like you, while everybody didn't quite know everybody, the culture was pretty homogenous.
My concerns weren't "selfish", but everyone I cared for was so similar to me that my own needs matched what I knew the country wanted. My needs were reasonably met by the politicians of the time, the needs of everyone I cared about were also reasonably met by politicians, so a conservative political leaning was ready to go. Small incremental changes to the status quo seemed like the correct style of government.
However that all changed when I went to get a degree. People in my class weren't carbon copies of me anymore. The people I made friends with had first languages other than English, they were LGBT, they had different religious beliefs, they had different skin colours. By expanding my social circle to include a wider group and interacting with different people, I started to understand what it was like to be someone other than the carbon copies of me I'd grown up with. The people I met with were also having their social circles expanded at the same time, which went well beyond just fellow students, it became possible to see other social classes than white middle class suburbs because I got to know the friends my new friends had grown up with.
Now my experience was Australian not American, however there are many parallels here. First friend with an Aboriginal identity I got an understanding of police racism and had a face to put on that problem. First non-binary friend I got an understanding of trans rights and now had a reason to vote that way. First lesbian friend I now had a reason to vote for gay marriage. First heart to heart with a domestic violence victim raped by her partner who had to make a tough choice and suddenly my Pro-Life upbringing melts away. Seeing a friend struggling to live on $40 a week and I want to raise the welfare payment.
That experience changed my social circles into a social web, and the echo chamber of lower-middle class white clones of me became a melting pot of different experiences and different struggles. I always cared about the people around me - just the people I empathise with are more varied now, and struggling with things I wouldn't of otherwise been aware of. The awareness of those struggles motivates my left-wing leaning.
Had I never gone to college I'd probably be a lot more politically conservative.
Existing answers pretty much cover the ground well, but I will answer a specific assertion in your question that is not actually correct either:
High school also teach similar topics as colleges, such as science, social science
That is not correct, either qualitatively or quantitatively.
One thing that most people discussing the topic usually fail to make clear (usually for lack of knowing this detail), that this education divide is not simply about "education". It is extremely strongly influenced by the kind of higher education you obtain, and who teaches you.
There are several things to unpack here:
Difference in outcomes between American (probably Western in general) colleges
Difference in subjects covered
Difference in teaching styles
Difference in ages of students
Let's cover them one by one.
To be blunt about it, it's not "college" that makes one support left wing ideology, it's a Western (in this case usually American) college.
The most drastic, clear proof of this is a natural experiment that happened in 1980-90s, when a large group of highly educated people immigrated into the USA - Russian Jewish community. I'll be self-plagiarizing details from my earlier Skeptics.SE answer:
60% of the demographics holds 5+ years of higher education.
This compares to 27% US population overall and to ~60% of all American Jews (src) (the latter serve as a good control group since 4th wave of USSR immigrants are similar in ethnic/religious composition - being overwhelmingly Jewish - AND similar education level).
Overall, the control group with very similar demographics is American Jews who tend to vote 70-90% Democrat in Presidential elections.
An overwhelming majority of educated immigrants from former Soviet Union are hard-anti-progressive (technically speaking, they usually vote "R", but if you go into nuance, most are libertarian-ish politically - most of them not so much as vote "for Republicans", as "against Democrats" - literally, if you ask, that is their line of thinking).
There isn't too much polling on it as the demographics isn't prominent enough, but what there is, supports this. The poll #s below were sourced from "Russian-Jewish Immigrants in the U.S:Social Portrait, Challenges, and AJC Involvement" By Sam Kliger, Director, Russian Affairs, AJC. Specifically, "Voting patterns 2000–08" section of "Political Views" chapter.
2004: 77% of Russian-speaking Jews in New York voted for the Republican incumbent GW Bush over his Democratic challenger John Kerry who got 9%
Similar pattern in 2008: McCain 65%, Obama 10%
In 2011, in special Congressional election to replace Anthony Wiener (NY Congressman who resigned prematurely due to a sex scandal), Russian Jews voted 90% for the Republican candidate (Note: "D" candidate was Jewish; "R" candidate was not, so this split was even more dramatic compared to baseline control group expectations and fundamentals).
Additional citations can be found in "The New Jewish Diaspora: Russian-Speaking Immigrants in the United States ..." edited by Zvi Gitelman, Zvi Y. Gitelman; p. 116 (but I wasn't able to find citable text, just search inside Google books). Their main source for USA seems to be Kliger as well, though.
I have not been able to find more recent polls/studies, but from anecdotal personal experience, most people in my demographics I know of are still just as much anti-DNC as they were 10 years ago, Obama did not help that in any way for reasons that are beyond the scope of this answer.
Now, as to the subjects covered.
Your question's statement "High school also teach similar topics as colleges, such as science, social science" is not actually accurate. College covers a lot of subjects NOT covered in high school.
From more extreme/esoteric (gender studies, music theory, dance interpretation, psychology), to specifics of given large fields (yes, you study "social sciences" in school, but you do not study political science or sociology).
Some of them may have less effect on political views (I'm pretty sure biochemistry may be on that end of the spectrum) and some (gender studies) may have more effect.
Evidence for this was presented in Sam's excellent Politics.SE answer:
This looks like a pretty decent study as far as studies found on the internet go. It measures liberalism vs conservatism as opposed to Democrat/Republican, but it's close to what you're looking for. (pdf)
It suggests that Engineering and Business majors tend to hold more conservative views on both social and economic stances, and that Bio/Lab, social sciences, and fine arts majors have more liberal views.
Moreover, there exists an important distinction is textbooks and curriculum and the teachers' freedom of what to teach.
As a general thing, teachers in school tend to be forced to teach to pretty rigorous set of standardized tests, teaching classes populated by kids who are not necessarily academically inclined. That means they spend 99% of their teaching time teaching the basics. In other words, even a teacher who may be inclined to teach "political views", consciously or unconsciously, would simply have trouble finding time to do so.
Secondly, a LOT of curriculum and textbooks are determined in a rather unified way (I won't go into details of how and why here, just accept that they are - from Common Core on curriculum side to state education boards on schoolbook side). College teachers have far, far more freedom to teach basically whatever they want as long as their college OKs it.
A high school child will typically live with their parents, and absorb the parental ideology at a higher rate (I can hunt down references for that).
Once a college kid goes off to college, they get to be separated from the parents, both literally and psychologically in terms of influence.
Also, college age is typically when ideological "rebellion" starts - that is not exactly a new, or American phenomena - this was the case back in 1960, and it was the case back in Reformation era Germany, it was the case in anti-Mullah rebellions in Iran in 2000s and in anti-Communist sentiment in Eastern Europe in 20th century. Why that is, I don't have a good explanation I can cite, but it does seem to be the pattern.
Basically, college kids tend to rebel against societal authority (not just parental one like school teenagers) - and that tends to lead one to be more "left leaning", to use a very crude and not always accurate label.
Now, if you couple that with largely-left-leaning professors, and the two forces reinforce each other, the views falling onto receptive ears and brains.
There have been a lot of answers so far to this question, but most of them never refer to any evidence.
One very simple hypothesis is the following. By the time people get to be 18, their personalities are fully crystallized. The college system in the US then acts to filter these people into two different broad categories, based on the intellectual and behavioral characteristics they already possessed at age 18. College selects for kids who were already good at arguing and critical thinking, and willing to act according to the norms of the white-collar workplace (e.g., turning in paperwork on time). As evidence of this, there is a test called the Collegiate Learning Assessment, which is designed to measure critical thinking ability. Among college freshmen, about 32% of the variance in CLA scores is explained by their SAT scores, and by the time they graduate, this drops only a tiny bit, to 30% (Klein, p.18). Gains in critical thinking over the course of a college education, as measured by the CLA, are rather small: 0.18 standard deviations over the first two years of college (Arum, p. 35).
So in the population of the US, we have some people whose critical thinking skills are low enough that they would believe, for example, in the birther conspiracy theory, which Donald Trump was one of the earliest and most influential supporters of. These people are less likely to go to college and graduate.
Some answers have suggested that going to college exposes people to more diverse people, which makes them less racist and therefore less likely to be Trump supporters. This is a two-step hypothesis, and the evidence doesn't seem to provide much clear support for the first step. The first few papers I turned up in a google search on this kind of thing (Fischer, two papers by Wodtke) did not seem to clearly support it. It is true that controlling for racism and sexism reduces the effect of education on Trump voting so much that the 2016 election looks like previous elections (Schaffner), but since there doesn't seem to be much evidence that education cures racism, this three-way correlation among education, racism, and Trumpism is probably not one in which education plays a causal role.
There is also a clear correlation between education and rural/urban lifestyles. For example, 21% of people in Mississippi have a college degree, while in California it's 33%. The US two-party system is very strongly correlated right now with urban vs rural, and it's therefore natural that it correlates with education. So again, there are strong, obvious reasons for education to correlate with voting behavior not because education causes voting behavior but because preexisting characteristics cause both.
So in summary, a college education correlates with voting against Trump, and also correlates strongly with a number of personal traits, but the evidence does not seem to support the notion that college education causes any of these things.
We should also keep in mind that college does not make people into civil libertarians (60% of college students say the first amendment requires equal time for opposing views on a college campus), and that both liberals and conservatives in the US are susceptible to conspiracy theories and pseudoscience (think anti-vaxxers, or Alice Walker promoting David Icke).
Arum and Roksa, Academically Adrift
Schaffner, B., MacWilliams, N., & Nteta, T. (2018). Understanding white polarization in the 2016 vote for president: The sobering role of racism and sexism. Political Science Quarterly, 133(1), 9–34., https://onlinelibrary.wiley.com/doi/full/10.1002/polq.12737
Immigration is a major cause for the difference in voting.
The reason for this is that immigrants (in this case South/Central Americans) often have lower educational levels then in the USA. So a person who paves roads for a living has more chances of losing his job towards cheaper labor from immigrants then for example a doctor or a lawyer.
But besides the extra competition in the job market they are also getting more competition in the house market making it harder to find affordable homes for low/mid income families.
But besides immigration there is also the basic border control issue that the Republican party (for example Trump's new wall) holds high. This is because of the criminal elements (Cartel influence that is ramping up to be major problem south of the border) that can cross the border is more inclined to affect the low to mid income households then higher income households that have better paying jobs due to their education.
So an anti open border political stance would make a party more popular among people without higher education.
Edit: To make it clear, I am saying that a lot of low to mid income households hold these fears and are voting Republican because of them. If those fears are justified is an other discussion.
Having a college degree is a proxy for being smart.
Pollsters cannot ask directly how capable someone is at critical thinking, rational analysis of arguments, understanding consequences of policy, or able to do simple fact checking on outright lies. However if you have a college degree it is more likely that you are at least capable of doing these things (since if you can't do them you are unlikely to get a college degree).
This is of course just a statistical correlation - there are plenty of people capable of all these who don't have college degrees, and some with college degrees who can't do them. And there are plenty of intelligent and educated people who do vote for Donald Trump. But the undereducated do vote for him disproportinately.
The President denies science
The current President denies science (on topics that matter in the current elections), and educated people may care more about that than less-educated people. This aspect applies specifically to the situation in the USA in 2020 and cannot be generalised to other times or places¹.
The current American president (2017-2021) is anti-science. He has denied the science on climate change as a "Chinese hoax", he has repeatedly contradicted science on the COVID-19 pandemic, spreads conspiracy theories and "alternative facts", and his administration has repeatedly silenced and attacked government scientists. With his anti-science "alternative facts" approach it is therefore no surprise that few scientists support him (perhaps unless their job depends on it). People with college education may be more likely to consider this a fault than those without. Although in general denying science can and does occur on all sides of the political spectrum², in the US in 2020 the President is particularly associated with it compared to the main opposition candidate, and in the 2020 election, this denial has a major impact on the major political topics of climate change and the COVID-19 pandemic. See also What caused the tendency for conservatives to not support climate change regulations? and Why is denying global warming associated with conservatives? for more on denial of environmental science in general and climate change in particular.
This is far from the only reason, but it contributes, and it is a factor that didn't apply in the past.
It is telling that the normally neutral Scientific American has endorsed a candidate for the first time in history (of course they also criticise left-wing candidates when their approach is anti-science, as they are not a political magazine; their agenda is to defend science as such, not a particular political ideology).
¹I am only aware of the federal situation. I cannot comment as to whether partisan denial of science applies to state-level elections in the USA in 2020.
²For examples on the left, one can name exaggerating the risks of genetic modification or nuclear fission power; for examples occurring on all sides, consider anti-vaccine campaigns or "electric allergy" and associated 5G protests. As far as I'm aware, none of those are significant election campaign issues in the 2020 United States Presidential elections.
Well answer is that we do not know since all social "science" "research" will be thinly veiled "stupid people vote Republican".
Colleges staff are extremely left wing, especially in social "sciences". According to Fox News:
A 2004 poll found that among sociology professors, 25 percent self-identified as "Marxist,” 49 percent identified as Democrats, and 5 percent as Republicans.
So in 2004 there were 5x more Marxists professors of sociology that Republicans...
So it could be a mix of people who go to college being attracted to leftist ideas, it could be the case of college education causing the shift, it could be the fact that major cities are where colleges mostly are(and people in cities are more left leaning), it could be that student loans make you more likely to want to redistribute some wealth...
We do not know, and for sure social "science" research will not tell us since only allowed result in social sciences is that Republicans are evil, Democrats are good.
The United States is theorectically a meritocracy, where people rise to the level of their talents, and where there are no hereditary aristocracies, or permanent underclasses. In practice, however, American society is highly stratified, with less class mobility than Americans tend to assume. One of the most important markers of class status, particularly among white Americans, is college education. The skyrocketing costs of college education make it difficult to access for the poor. However, it's also the most reliable route out of the underclass for those who ARE able to achieve class mobility (providing mastery not only of content knowledge, but also of a range of class-associated cultural values and signifiers).
In other words, college degree can reliably be used as a proxy for a constellation of stable cultural and socioeconomic differences, which, in turn, impact what people perceive their interests to be, and whom they identify with, and thus, whom they vote for.
What was asked:
Why does having a college degree or not make a difference among white Americans?
Stating the question that way seems to imply that having a degree changes who you are and what you believe. I don't think OP was implying that, although there are answers that have gone that direction. Instead I think what is being asked is:
Out of all people with college degrees why are there more people voting Democrat?
I will try to answer this restated question.
Republicans value college differently from Democrats
One reason why there is a difference in the voting of college graduates is because there is a difference in the number of conservatives that attend and finish college. Republicans are less likely to go to or finish college, not primarily because of their intellectual capability, but because of differences in preference or the value they see in their time at college.
Percent of students that identify as conservative
In the UC system in California in 2020 about 10% of students identify as somewhat to very conservative. If you compare this to the percent of overall Republican registered voters in California which is 24%, you can see there is a discrepancy.
From this article that says going to college does not make you more liberal, a study found the following:
the tendency of college graduates to be more liberal reflects to a large extent the fact that liberal students are more likely to go to college in the first place.
Purpose of higher education
Many Republicans may not have the same views on higher education as Democrats. One such example is the purpose of going to college. A majority of Republicans believe the main purpose is to teach specific skills and knowledge that can be used in the workplace while Democrats believe that college both teaches skills and helps an individual grow personally and intellectually.
College campuses have a political agenda/tilt
Many Republicans believe that most college campuses are liberal, and push their liberal agenda onto the students thereby converting them. This argument about conversion is largely proven false by some studies, but this is still a concern for most Republicans. This means a Republican parent might think their child will be intimidated into being liberal. For the same reason, a liberal parent would avoid sending their child to an overtly conservative school unless they had an overriding reason such as religious beliefs.
College students that identify as Republicans hide their political views in class or on projects for fear of getting bad grades. Would you want to continue going to a school where you have to hide who you are?
Colleges are not preparing students for their career
A majority of Republicans believe that colleges are not doing a good job at preparing graduates for the workforce. In addition, a majority of Republicans think gaining skills for your career is more important than any other aspect of college.
Parents that feel this way might instead choose to send their kids to a trade school or encourage them to get a job directly. If a conservative student felt that their college was not preparing them to get a job they'd likely get a job and drop out rather than rack up debt or spend money on college.
Most of this response is rooted in the information found in this PEW article, there are many other studies and articles available to back up the items listed above.
UC Voter Participation Survey https://ucop.edu/institutional-research-academic-planning/_files/ug-voter-participation-at-uc
California Voter Registration Report https://www.sos.ca.gov/elections/report-registration/60day-gen-2020
the majority of non-college-degree white people support Trump's reelection, while the majority of college-degree white people don't
This may be true but many college-educated Trump supporters simply won't admit to it because of the backlash they would receive. I suspect this may bias a lot of polls.
Many lecturers in American colleges are radical Left, especially in the "softer" subjects. The non-academic staff are very reluctant to challenge any "politically correct" views for fear of repercussions.
As a result, in the current climate, conservative students have to be careful about revealing their beliefs and affiliations for fear of being branded fascists/racists/white-supremacists and every other "ist" that the left can throw at them. This can result in 'cancelling' and doxing, not to say just being screamed at in the corridors.
Even mildly right-leaning students are aware of this.
Here's an example of a student who lost her university job as a result of asking students to consider both sides of a particular incident.
A few relevant divisive points not made by other answers:
Colleges additionally infuse students with just enough superficial modern economics to make the now confused student more susceptible to politicians who mostly abandon working class laborers, (i.e. the parents of many students), under cover of economic rationalizations.
Modern colleges didactic anti-racism and economism are usually too superficial to make persuasive evangelists of most students. A lone student returning to their home town wishing to enlighten their friendly non-college peers may fail miserably, and in despair either retreat to a city, or settle for a less influential marginalization, whether clubby or cringing.
Colleges in 2020 often come with substantial and resilient debts. Severely indebted former students comprise an interest group with different needs than blue-collar workers.
A candidate who would forgive student loans is much more appealing, as is any candidate who supports professional accreditation standards financially beneficial to graduates.
A student or graduate too indebted to start a family may, contrary to prior generations, feel an unfriendly and envious resentment towards their blue-collar former peers who have families.
Meanwhile both the presence of and the lack of a college education evidently make those respective groupings more easily manipulated by divisive yet flattering targeted propaganda, helping each group experience a righteous indignation toward a ridiculous opposition.
The outcome of this inarticulate learning, credentialism, debt bondage, propaganda, etc. is that all of these set students and their blue-collar former peers at odds, so that a profitable dialogue seldom occurs, and instead America puts out its lawn signs.