Quality challenger emergence under the top-two primary: Comparing one-party and two-party general election contests

Prior research finds that the emergence of a quality challenger is one of the most important factors predicting whether incumbents will be vulnerable. Reformers in California and Washington envisioned that the top-two primary reform would increase electoral competition by allowing for general election contests that feature two same-party candidates in safe districts. In this research note, I investigate the degree to which these expectations have been fulfilled by looking at the prevalence of quality challengers in U.S. House contests. I compare one-party and two-party general election contests, finding that incumbents are significantly more likely to face a quality challenger from the same party than from the opposite party, all else equal. In contrast, when both states used traditional primaries prior to reform, incumbents were no more likely to face a quality challenger in the primary election than in the general election. Findings reveal a key way in which the top-two primary may fulfill reformers’ expectations and complement our understanding of how electoral institutions condition challenger entry decisions.