Every now and then, a question like this one gets answered by an explanation of the Southern Strategy. Now, I recently saw a quote attributed to Abraham Lincoln saying that the Democrats of his day were the Whigs of some time prior, and indeed one answer here states that the parties have switched several times.
So I wonder: How many times has a large coalition shift between the two major parties of the United States happened?
The answer to this question is in the term "Party System". A party system is some stable group of political parties, poised in equilibrium - sometimes one wins, sometimes another does, but they generally represent the same things, are built from the same coalitions, and have their influences wax and wane but they can be viewed as being roughly the same thing over time.
...except when the Party System changes. And it's those breakpoints that you're talking about. With 1968 as the decisive year, the US transitioned from its 5th Party System (with Democrats in an uneasy coalition between northern liberals and southern 'Dixiecrats') to the present-day 6th Party System. Check out the Wikipedia article:
Roughly speaking, in the US, you had these:
(1) Federalists (Hamilton/Madison) vs Democratic-Republicans (Jeffersonians)
(2) Whigs (Henry Clay) vs Democratic-Republicans (Jacksonians)
(3) Republicans (born 1850, took power with Lincoln in 1860) vs Democrats (the South), through Reconstruction till 1896
(4) Progressives (Democrats, frontier/rural in South and West) vs Republicans (Northeast / business interests), till 1932
(5) Democrats ("New Deal", via FDR), pressing social-welfare and an expansionary State vs Republicans preferring small government and fiscal austerity, till 1968
(6) Democrats pivoting to a civil rights focus, earning the African-American vote en masse as Republicans' southern strategy seized the South for generations since.
(historians and political scientists disagree on precise start/end points, and might combine #3/#4 or #4/#5 into a single era.)
This leaves aside, of course, any political trends prior to the revolutionary war, or even in the Articles of Confederation period. But for our purposes, post-Constitution, the answer to your question is probably Five.
This is a myth that has been brought up several times in history. However an examination of history shows that this is not the case. The confusion comes from two factors: Social changes over time and the trying to fit in the ideologies of liberalism and conservatism into the two parties in discussion. In the end the political base of the parties have remained the same. I will focus on the so-called southern strategy that took place around 1966
Political Basis of the Democrat Party: Strong State government in comparison to the federal government. However the government should have a strong relationship with the citizens.
Political Basis of the Republican Party: Eliminating Slavery, weak government all around but still having a stronger state government in comparison to the federal government
The confusion of trying to fit the ideologies (Conservatism and Liberalism) exclusively into one party or the other is where most lay people get in trouble.
Like most groups of people, there is not one homogeneous belief in either party. There have been and always will be conservative democrats and liberal republicans. This has not changed, nor has any group has had more influence in the leadership of either party. By the time you get to the leadership level, ideologies have been compromised so often that there appears to be no difference...
Over time technology advancements and the ebbs-and-flows of economics and social norms change so that it is not possible to judge the ideologies of the past with the standards of the current era.
The main point of the case for the "switching" comes from the misinterpretation of the so-called "southern strategy". Democrats accuse republican political strategists of racially ignoring southern blacks and getting the vote of racist southerners by appealing to the racism of the south.
This ignores many of the realities of how the real world works, populations and social norms are never static, especially in the US where freedom of movement is one of the main pillars of American culture. What I mean is that it is too easy to ignore the numerous migrations of people with specific party loyalties. To summarize, in the century since the civil war to the days of the southern strategy, southern democrats moved north, northern republicans moved south, conservative blacks moved south and then after WW2 moved back north to urban areas and became democrats with conservative leanings and POOR.
Factors of migrations:
There were other social/cultural issues that changed the make up of the parties. And I still don't mean Southern racist democrats deciding that the Republican party was more welcoming nor northern Republicans seeing that their ideals were better served by democrats.
Social/Cultural factors:
AS a result, it is strange that the conclusion of the so-called southern strategy focuses on ignoring blacks when what it actually was the ignoring of the entire northeast, west coast and concentrating on middle class people in the mid west, plain states and south.
Both Parties divided that population by how they would vote. The Republican strategy was socio-economical while Democrat stuck with the racial/ethnic component. This strategy remains the same to this day. You will see that Racial/Ethnic divide is still Democrat while middle class is still the base of the republicans.
democrat should be democratic).