Do Primary Elections Exacerbate Congressional Polarization?

Anthony Fowler, Professor of Political Science, University of Chicago
Encina Hall West, Room 400

Many argue that primary elections increase polarization in the U.S. Congress. We test this hypothesis by leveraging variation in primary election dates and candidate filing deadlines across states. Implementing differences-in-differences designs that account for idiosyncratic differences between each member in each Congress and each bill by party, we test whether members vote differently before or after their state’s filing deadline or primary election date. Members of Congress do not appear to cast more partisan or ideologically extreme votes to deter primary challengers, but they do cast more extreme votes during their contested primary campaigns. However, the substantive magnitude of this effect is small, explaining approximately one percent of congressional polarization. We further find that the polarizing effect of primary elections is greater in the Senate, smaller on party-priority legislation, greater for more moderate members, and smaller in states utilizing nonpartisan primaries.


Anthony Fowler is a Professor in the Harris School of Public Policy at the University of Chicago. His research applies econometric methods for causal inference to questions in political science, with particular emphasis on elections and political representation. Specific interests include unequal political participation, electoral selection and incentives, political polarization, and the credibility of empirical research. He is an editor-in-chief of the Quarterly Journal of Political Science, an author of Thinking Clearly with Data, and a host of Not Another Politics Podcast.