We study the conditions under which estimating the incumbency advantage using a regression discontinuity (RD) design recovers the personal incumbency advantage in a two-party system. Lee (2008) has introduced RD as a method for estimating what is generally considered the "partisan" incumbency advantage. We present a causal model with some simple but plausible assumptions that allows RD to be used to estimate the "personal" incumbency advantage, as an alternative to sophomore surge, retirement slump, and other commonly used measures. We estimate the incumbency advantage using our model with data from U.S. House elections between 1952 and 2008. Using the assumptions of our model, we also explore the estimation of the incumbency advantage beyond the limited RD conditions where knife-edge electoral shifts create the leverage for causal inference.