Electoral reforms affect legislative outcomes by influencing incumbent legislators’ behavior, new entrants’ behavior, and the probability that incumbents are replaced with new entrants. Empirical work on electoral reforms and polarization has focused on new entrants’ behavior. We employ a simple decision theoretic framework with partial incumbent policy persistence and spatial voting to examine the three channels jointly. We show that a reform designed to encourage ideological moderation produces larger effects on polarization when the reform is implemented than when it is removed. The key insight is that implementing a moderation-inducing reform generates a set of challengers who are more likely to defeat incumbents while the incumbents are more likely to win reelection when the reform is removed. We then empirically examine how elections and legislative polarization respond to unlimited PAC contributions in state legislatures. Examining incumbents’ decisions to stand for reelection, the electoral performance of incumbents who do run, and partisan polarization, we find empirical support for our predictions.