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I have been thinking about the Democratic Party and whether or not its members are more numerous than the opposing faction.

Evidence to suggest this is the case:

  1. This party is expected to win the popular vote for president seven out of eight times since 1992. Please don't say "this hasn't happened yet". If this bothers you, say 6 out of 7.
  2. The party has received 51.9 percent of the votes cast in presidential elections from 1992 to 2016 for it or its opponent, the Republican Party. This shows that 2012 was the mean election in popular vote as of 2016.
  3. Party registration in states that register by party says this same thing.
  4. Trump's approval has not gone above 50 percent ever as president on 538.
  5. A plurality of Americans consistently supported impeachment by 2 to 5 points while it was happening.

This suggests that the partisan lean the American electorate is about D+4. I believe that it might be closer to D+5 now for various reasons and the fact that 2012 was the mean result. This can get a little bit fuzzy because of independents.

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    I like this question, but there are some subtleties being glossed over here: a fair number of citizens are apolitical, a fair number of voters are not registered to a particular party but end up voting for one of the two main parties (and not necessarily always the same one!), national and state/local elections have different mixes, as Republicans control and have controlled a majority of Governorships for some time, etc. But yeah, after taking those caveats it seems that Democrats are indeed more populous/popular, which begs the question of why Republicans are so relatively successful. – Jared Smith Aug 3 at 14:36
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    In political science, it's usual to make a distinction between members of a party and its electorate. This question appears to conflate the two, but that's understandable given the weak nature of US parties. – MSalters Aug 3 at 15:31
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    The party system is becoming stronger. This is due to ideological and geographic sorting. The Democrats have a liberal and moderate platform, while the Republicans are predominantly conservative. Straight ticket voting is rising in most parts of the US. That allowed Democrats to win the popular vote for governor by almost 2.9 million votes in 2018 despite losing states they should never lose in a stronger environment at that level though those states are solidly Democratic and progressive at the federal level. – Bitecofer Aug 3 at 15:36
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    None of your evidence points is actual evidence for party affiliation. If you want to know that, check the voter registrations. – Robert Harvey Aug 3 at 15:38
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    @RobertHarvey: None of your evidence points is actual evidence for party affiliation. If you want to know that, check the voter registrations. The OP's #3 is exactly that. And voting registration doesn't necessarily indicate that the voter considers themself to be a member of that party. I don't like either major party, but until California had jungle primaries, I would always register as one or the other so I could have some influence in the primary. – Ben Crowell Aug 4 at 0:07
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It really depends on how you define 'members', but both early registration totals and opinion polling suggest that there are significantly more Democrats than Republicans. For example, Ballot Access News has collated the early 2020 registration totals from 32 states, and presents the figure of 45,715,952 Democrats compared to 33,284,020 Republicans in its March 2020 edition, which translates to a two-party advantage of D+15. However, this figure needs to be taken in context - similar figures from 2016 also gave a similar early advantage of D+15.

If we look at opinion polling, Gallup has collated party affiliation polls back to 2004. The most recent poll at the time of writing gives a D+11 advantage. Looking just at the net Republican/Democrat advantage, ignoring Independents, we can create the graph below - with positive percentages representing a Democrat lead, and negative percentages representing a Republican lead.

enter image description here

If we also take into account the lean of the Independent respondents, the most recent poll gives an advantage of D+14, with the respective graph as below.

enter image description here

Using data on party identification from the Cooperative Congressional Election Study 2018, after weighting the survey data to be representative of adult Americans, this gives a D+10 two-party advantage when respondents were given a three-point (Dem/Ind/Rep) scale, and a D+7 advantage on a more finely-grained seven-point scale.

When the responses are instead weighted to be representative of registered voters only, these two-party advantages remain very similar - D+11 and D+7 respectively.

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  • I think D+4 could've been an underestimate. The median year for this was 2004, which is kind of ironic. It seems likely that it progressed up to D+7 now. – Bitecofer Aug 3 at 15:33
  • 2016 was D+2. This was presumably because Clinton was widely viewed as a weak candidate. Polls show the voters being D+9 just like in the 2018 House. Don't talk about any other 2016 federal elections. – Bitecofer Aug 3 at 15:41
  • If you gave me those graphs as the result of scientific research I would say it doesn't support the conclusions because there's a larger variable you haven't controlled out yet because the noise in the graphs exceeds the signal. – Joshua Aug 4 at 21:47
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To give a theoretical perspective on this:

Going by your question, I assume the following interpretation of terms matches your intentions: A Democrat is somebody who is more likely to vote Democrat than Republican (and vice versa). I here introduce the notion of probability to account for effects of individual elections such as preferences for particular candidates, incumbency bonus, scandals, etc. Instead this is about general party preferences.

The US is a textbook example of Duverger’s law having a two-party system as the equilibrium state of an plurality voting system. In such a two-party system, there is a trend of parties (or candidates) to adjust their positions such that they have roughly equal chances of winning: If a party (or candidate) is currently on the losing side of things, it has to move towards their opponent to gain more voters; if a party (or candidate) is winning, it has no big incentive to move towards the opponent. By the way, something similar happens in multi-party systems, where there is a trend towards an equilibrium between the left- and right-leaning coalitions.

Now, there are other effects, such as regional variations, politicians following motivations other than winning (such as ideals), candidates and parties misjudging their current chances, etc. However, most of these effects act on short time scales, while being a Republican or Democrat¹ changes more slowly and thus many of these effects are averaged out. The thoughts going into this are very similar to the underlying assumptions of the median-voter theorem. Essentially, my claim is an inversion concluding that the divide between two parties represents the median of the electorate (or more precisely, the point of equal chances to win an election). Kramer’s A dynamical model of political equilibrium also seems to tie into this on a higher-dimensional political landscape. There are studies showing that there is always a fixed distance between quantified political positions of the two main presidential candidates in the US (except for 2016), but I fail to find it right now.

So, in an plurality voting system, there is an equilibrium where both parties have equal chances of winning. Now, let’s turn to the specifics of the US. It is clearly close to that equilibrium, so if its voting system and regional structure were free from any biases (within the confines of plurality voting), we would expect that the electorate would be split¹ in half between the big parties. However, US voting systems feature considerable biases towards Republicans, such as general effects of geographic distribution, rural low-population states having more political weight, gerrymandering, various forms of disenfranchisement (also see this question). As a consequence, there must be more Democrats than Republicans¹ to achieve the aforementioned equilibrium.


¹ as defined in the first paragraph

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Yes, at least according to numbers Wikipedia sources to Ballot Access News. The Republican Party is listed as having approximate 33.2 million members, while the Democratic Party is listed around 45.7 million.

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