Is Political Ideology Stable? Evidence from Long-Serving Members of the United States Congress

This study extends the political science and political psychology literature on the political ideology of lawmakers by addressing the following question: How stable is a legislator's political ideology over time? In doing so, we employ Nokken-Poole scores of legislators' political ideology for members of the United States (U.S.) House of Representatives and the U.S. Senate who were elected prior to the 103rd Congress that began in early 1991 and who served consecutively through the 115th Congress, which ended in early 2019. Results from individual time-series estimations suggest that political ideology is unstable over time for a sizable portion of the members of both major political parties who serve in the U.S. Congress, while analysis of the pooled data suggests that, after accounting for inertia in political ideology and individual legislator effects, Republican legislators become more conservative over time. These results run somewhat counter to the finding in prior studies that the political ideologies of lawmakers and other political elites are stable over time.