It is well established that legislators from highly professionalized bodies are more likely to win reelection than members of less professionalized legislatures. We find that the effect of professionalization on incumbent electoral success is far more pervasive. As the level of professionalism of a legislature increases, the effects of external political and economic forces (such as coattails from higher level elections and national economic conditions) on a legislator's chances for reelection diminish in strength. This implies that legislative professionalization promotes institutionalization by establishing boundaries that insulate members from external shocks. We reach these conclusions by specifying and testing a district-level model of state legislative election outcomes, using as dependent variable the probability that an incumbent will win reelection. The model is estimated with probit using data for more than 42,000 state legislators from 1970 to 1989.