In this article, we use a regression discontinuity design to estimate the causal effect of incumbency on campaign contributions in the U.S. House and state legislatures. In both settings, incumbency causes approximately a 20–25 percentage-point increase in the share of donations flowing to the incumbent’s party. The effect size does not vary with legislator experience and does not appear to depend on incumbent office-holder benefits. Instead, as we show, the effect is primarily the result of donations from access-oriented interest groups, especially donors from industries under heavy regulation and those with less ideological ties. Given the role of money in elections, the findings suggest that access-oriented interest groups are an important driver of the electoral security of incumbents.