Presidential primary outcomes as evidence of levels of party unity

After the early primary contests, Joe Biden’s candidacy looked like it was in trouble. However, by April 8 Bernie Sanders conceded marking the earliest end of an open primary since 2004. Despite the large number of entrants, the Democratic Party was relatively unified. To show this is the case, I compare the 2020 Democratic primary to both parties’ primaries in 2016. The relatively higher unity displayed by the Democrats gives an indication of where the parties may be headed in the years to come.

Let us begin by considering 2016 for both parties. Each contest extended into the summer. However, the contests were quite different as Hillary Clinton faced a single opponent while the field was much more divided among Republicans. For much of the race, there was not one clear alternative to Donald Trump.

Like the 2016 Republican Primary, the 2020 Democratic Primary consisted of a record number of entrants, thus making for an interesting comparison. Here too we observe relatively more unity within the Democratic Party: Biden won support of 51.8% of primary voters, while Trump won 44.9%— the lowest of any nominee since Michael Dukakis in 1988. Moreover, the votes were spread more evenly across several candidates in the Republican primary. We can compare the three contests in Figure 1, which displays the vote share for each candidate who received at least 1%.

Figure 1. Vote share for candidates receiving 1% or more of the vote

Another way to examine the unity of the two parties involves utilizing a measure common in other fields to gauge diversity such as E. H. Simpson’s Measurement of Species Diversity.” published in Nature in 1949. For our purposes, I have recoded it so that higher values mean more “unity”—it would be 1.0 if all voters voted for the same candidate (the minimum value is 0.0 if votes were evenly divided across many candidates). In 2020 Democrats received a score of 0.5, compared to 0.45 for Republicans in 2016. To put recent contests in perspective, Figure 2 traces this measure back to 1976 for both parties. The open circles represent years with competitive primaries, the black circles represent years when a sitting president was eligible for re-election—for example, President Obama in 2012 and President Trump in 2020. The left-hand panel displays the Democratic Party and the right-hand panel the Republican Party. In each panel, the line represents the trend for open primaries.

Figure 2. Unity for primary voters by party, 1976–2020

Two conclusions emerge from the figure. First, the primary electorate in both recent Democratic contests displayed more unity than the 2016 Republicans. Second, at least by this measure, the two parties have been moving in the opposite direction since 1976: The Democrats have become more unified and the Republicans less so. Another way to look at the data is to note that even though there was an unprecedented number of entrants competing for the Democratic nomination in 2020, the party was only slightly less unified than in 2016 and was slightly more so compared to 2008. There is little evidence that the Democratic primary electorate is hopelessly divided.

The data presented here is consistent with other evidence that indicates the Republicans are more internally divided than Democrats—both among elected officials and the public. A factional candidate like Trump was able to narrowly win the nomination without majority support in the party, while in a similarly large field in 2020 the Democrats coalesced rather quickly around a consensus candidate. This lack of internal cohesion among Republicans may also help explain the lack of governing success that Republicans have had this century—for example, beyond tax cuts and judges, the Republicans have little to show from their two years with unified control of government and it would be difficult to classify George W. Bush’s presidency as a success.

Whether these trends continue moving forward is, of course, an open question. And while time may prove me wrong, my hunch is that despite some of the handwringing that has emerged immediately following the election about the Democratic Party’s “underperformance,” chances are that over the next few election cycles the party remains relatively unified. As for the Republicans in 2024, it seems reasonable to imagine that the internal divisions will continue to exist, and that a protracted and fractious fight is reasonably likely scenario.