In 2020, an average of 18.3% of age-eligible voters cast ballots in congressional primaries. This trends similarly with recent midterm election years: turnout of eligible voters was 19.9% in 2018, 14.3% in 2014, and 18.3% in 2010.
Turnout can fluctuate dramatically depending on both environmental and policy variables. Environmental factors include whether there are competitive or statewide races, high-profile ballot initiatives, and/or media coverage that draws awareness to the election. Top policy factors include the type of primary and which voters are allowed to participate, the date of the primary, whether or not the state uses runoff elections.
Data collected during the 2020 congressional primaries found, on average, voter participation in California (34.4%) and Washington’s (44.8%) nonpartisan congressional primaries to be much higher than an estimated average 18.3% turnout rate in partisan primaries. A Bipartisan Policy Center report also found that total ballots cast in Washington and California in the three most recent midterm elections exceeded national averages (see Figure 11). In his 2020 study, which controlled for other factors, scholar Seth Hill found nonpartisan primaries responsible for a 6% increase in voter turnout rate in primary elections.
One consequence of low turnout primaries is that incumbents in Congress must appeal to a small and disproportionately partisan sliver of the electorate to get reelected, and will adapt their behavior in office to do so.
According to scholars at the R Street Institute and the Brookings Institution, who conducted a thorough review of the academic literature, find the fear alone of being “primaried” prompts members of Congress to adapt in three main ways:
Research using a novel dataset on partisanship and bill co-sponsorship found a competitive primary challenger from the ideological extreme decreased the rate at which Democrats and Republicans cosponsor bills with members of the other party, and also found the increased threat of an ideological primary challenger accounts for about one-fourth of the rise in partisanship that occurred from the 1980s to the 2010s,
But some evidence suggests the top-two primary has reduced ideological extremity among legislators compared to those elected in closed primary systems. In a 2020 study spanning U.S. House elections from 2003-2018, newly elected members from top-two primary systems had ideology scores that were more than 18 percentage points less extreme than closed primary legislators.
Academic literature comparing the ideology of primary voters and general voters is sparse. According to the most recent exit polling on congressional primary voters, the largest ever of which was conducted by the nonpartisan Brookings Institution in both 2016 and 2018, primary voters self-identify as strong partisans.
During the 2018 midterm primaries, the Brookings Institution conducted exit polling of primary voters, with a sample of 13,372 voters across 20 congressional districts where incumbents faced primary challengers.
The resulting study by Elaine Kamarck and Alexander Podkul found that primary voters overwhelmingly identified as “strong Democrats'' and “strong Republicans” (see Figure 4). They also found that primary voters were much more likely to identify as liberal or conservative than as moderate (see Figure 5). The polarization was asymmetric, though, as the number of “very conservative” Republicans far outpaced the number of “very liberal” Democrats.
There are currently three variations of nonpartisan primaries in practice:
In 2020, more than 40% of districts nationwide (151) had no competition in the dominant party’s primary, while just seven of the 63 congressional districts (11%) with nonpartisan primaries had only two candidates. In more than half of the nonpartisan primaries, voters had a choice between at least five candidates.
Evidence suggests after adoption of nonpartisan primaries, states observe increased electoral competitiveness. California and Washington far outpace the national average in both the share of contested primaries and the share of primaries in which incumbents face a challenger: they also have more incumbents running per district.
As of April 2022, 55 cities, counties, and states are projected to use RCV for all voters in their next election. These jurisdictions are home to approximately 10 million voters, and include 2 states, 1 county, and 52 cities. Military and overseas voters cast RCV ballots in federal runoff elections in 6 states.
More than 10 million voters are barred from voting in primaries in the nine states that have closed primaries: voters must be registered members of the party to participate.