Congressional Careers: Service Tenure and Patterns of Member Service, 1789-2021 [Updated January 5, 2021] [open pdf - 1MB]
From the Introduction: "This report presents data on average Member tenure over time, analyzes several factors that affect tenure in any given Congress, and examines historical patterns of congressional service, including the distribution of years served within each Congress, and the cross-chamber experience of Representatives and Senators. [...] Two factors that affect average years of service are examined in this report: the percentage of Members choosing to seek reelection, and the success rate of Members who do seek reelection. The findings presented here align with scholarly assessments of congressional history, which conclude that during the early history of Congress, turnover in membership was frequent and resignations were commonplace. Many lawmakers in the 18th and early 19th centuries might be characterized as 'citizen legislators,' holding full-time nonpolitical employment and serving in Congress on a part-time basis for a short number of years. During the 20th century, congressional careers lengthened as turnover decreased and Congress became more professionalized. The report also examines two further issues related to Member tenure, including the distribution of Member service over time, as well as Members' cross-chamber experience. Although the average tenure of Members has increased since the late 19th century, a substantial portion of Representatives and Senators in recent Congresses have served for six years or less in their respective chambers. With respect to cross-chamber experience, while a small proportion of Representatives have historically had previous Senate experience, a sizable percentage of Senators throughout congressional history have had previous House experience."
CRS Report for Congress, R41545
Congressional Research Service: https://crsreports.congress.gov/