How Divisive Primaries Hurt Parties: Evidence from Near-Runoffs in US Legislatures

In many democracies, parties use primary elections to nominate candidates. Primaries may help select quality candidates, but they can expose flaws and offend losing candidates’ supporters. Do divisive primaries help or harm parties in the general election? Existing research is mixed, likely because of issues of selection and omitted variables. We address these issues by studying southern US states with runoff primaries—second-round elections that, when triggered, create more divisive primaries. Using a regression discontinuity design, we estimate that a runoff decreases the party’s general-election vote share in the House and Senate by approximately 6 percentage points and decreases the party’s win probability by approximately 21 percentage points, on average. Opposing results in southern state legislatures suggest that divisive primaries are damaging when salience is high but beneficial when it is low, a pattern we speculate is driven by the competing effects of information in high- versus low-salience primaries.