Are politicians ideologically rigid, or do officeholders adjust policy strategically for electoral purposes? This paper sheds new light on this longstanding question by studying how U.S. House incumbents alter their roll call voting record prior to elections depending on their challenger’s platform. Estimating non-incumbent candidates' policy positions using pre-primary transaction-level campaign finance data, I classify as extremist the more liberal (conservative) of the top-two candidates in Democratic (Republican) challenger primaries. Leveraging a regression discontinuity design, I exploit the quasi-random assignment of incumbents to moderate or extremist challengers by close primary elections of the incumbent’s opponent party. I find that incumbents alter their roll-call voting record depending on their opponent’s position, committing to a more moderate policy when running against an extremist compared to a counterfactual moderate. Consistent with strategic responsiveness to electoral incentives, policy adjustment to challengers is confined to re-election seeking incumbents and to incumbents defending a seat in a competitive district. I provide suggestive evidence that incumbents' reaction to challengers is conditioned by the presence of third candidates, and reflects a trade-off between persuading swing voters at the center and mobilizing core supporters. Importantly, incumbents' adjustment is not driven by a valence advantage of moderate over extremist challengers but by incumbents’ reaction to opponents’ policy positions, suggesting strategic complementarity of policy platforms
