Identifying the Source of Incumbency Advantage through a Constitutional Reform

This study provides one of the first causal estimates of both the personal and partisan incumbency advantages. Using data on six local elections taking place during the last 20 years in 278 municipalities in Portugal, it relies on a reform introducing mayoral term limits as a natural experiment that creates exogenous variation on the incumbency status of officeholders while holding the incumbency status of the party constant. A new methodology combining two quasi‐experimental methods, the regression discontinuity and the difference‐in‐discontinuities designs, allows for a credible estimation of the independent personal and partisan returns to incumbency. Results causally identify the personal effect as the driver of the incumbency advantage.