Biology
Why Party Primaries Push Candidates to the Extremes
Quick fact
In the 2020 Democratic primary, only about 9% of eligible voters cast a ballot in the Iowa caucuses, and these voters were significantly more liberal than the average Democrat. Similarly, Republican primary voters in 2016 were more conservative than GOP general election voters, helping propel Donald Trump's rise.
Why this is interesting
You might think that to win an election, a candidate should appeal to the most voters. But in many US elections, the real competition happens in a tiny primary, where only the most devoted partisans vote—and that changes everything.
Read the full explanation
Understanding Why Party Primaries Push Candidates to the Extremes
Imagine you are a candidate for the US House of Representatives. Your district is drawn to be safely Republican or Democratic. In such a district, the November general election is almost a formality—the winner of the primary is almost guaranteed to win the seat. This means the only election that really matters is the primary. Who votes in primaries? Not the average citizen. Primary turnout is typically low, often under 20% of eligible voters. Those who do show up tend to be the most politically engaged and ideologically committed members of the party. They are the activists, the donors, and the party loyalists. They care deeply about issues and want a candidate who shares their uncompromising views. So, to win the nomination, a candidate must appeal to this narrow, intense group. That means taking more extreme positions—more liberal for Democrats, more conservative for Republicans—than they would need to win the general election. The same logic applies even in non-safe districts, but the effect is most powerful where the primary is the decisive contest.
A deeper explanation
The underlying mechanism is a combination of electoral competition and voter preferences. The median voter theorem tells us that in a general election with two candidates, they tend to move toward the center to capture the median voter. But in a primary, the 'electorate' is not the general public; it is a self-selected subset of the party's most loyal and ideological members. This creates an incentive for candidates to shift away from the center. This shift is reinforced by several factors. First, party activists control key resources: endorsements, campaign volunteers, and donations. They can reward candidates who are ideologically pure and punish those who compromise. Second, candidates may be 'primaried' from even more extreme challengers if they are seen as too moderate. Third, in districts that are safely gerrymandered for one party, the general election poses no threat, so there is no pressure to appeal to moderates. The result is that primary elections systematically select for candidates who are more extreme than both the party's general election voters and the district's median voter. Once in office, these candidates must still face the general electorate, so some may moderate in their public messaging, but their record and policy positions remain anchored toward the extreme, contributing to overall polarization.