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This fall, the Democrats face a challenging US Senate map, defending 25 seats to the GOP’s nine. For some on the left, one ray of hope has been the retirement of four Republican Senators, leading to open seat contests. In new research, Hanna K. Brant, L. Marvin Overby, and Theodore J. Masthay suggest that this optimism may be misplaced. Studying nearly a century of Senate elections, and accounting for a number of other factors, they find that Republicans have a roughly ten percent advantage over Democrats in winning open Senate seat races.

As the 2018 midterm elections approach, pundits, politicians, and the public speculate about the possibility of a Democratic takeover of both chambers of Congress. In this age of marked incumbency advantages, this conjecture is driven in part by the partisan disparity in announced retirements: 48 Republicans versus 21 Democrats in the House; four Republicans (Corker of Tennessee, Flake of Arizona, Hatch of Utah, and Sessions of Alabama) versus one Democratic (Franken of Minnesota) in the Senate.  In new research, we offer a cautionary overview of the data, at least on the Senate side.   Using an original dataset, we examine vacated Senate seats from 1919 (the first year of an entirely popularly elected Senate) to 2014, focusing on the ability of parties to hold these important legislative offices following retirements. Our findings demonstrate that – net of other effects – Republicans are systematically advantaged in this regard, which has important implications for understanding the partisan balance of power in the closely-divided upper chamber.

In an era of substantial incumbency advantages, open seats are “where the action is.”  Legislative retirements present the party that is not in power with more realistic chances of capturing seats that they might not have held in decades.  Conversely, they present the party of the retiring incumbents with added hurdles to retaining these seats, seeking to compensate for the lost “personal vote.”  It is worth noting that retirements have been a significant source of turnover in the Senate.  Between 1919 and 1972, they accounted for 39 percent of all departures from the Senate; since the 93rd Congress they have accounted for almost two-thirds of all changes in Senate composition.  Republicans’ lack of internal divisions and intra-party discipline, it turns out, mean that they are better better equipped to retain open seats because their party’s structure makes it easier to focus resources on retaining open seat when their members retire.

Credit: FEMA/Bill Koplitz.

Over the history of the popularly elected Senate, Democrats hold their vacated seats approximately 61 percent of the time (104 of 171 instances), while Republicans experienced success over 71 percent of the time (105 of 148). We break down these numbers by decade in Figure 1 below, which shows that the partisan difference is relatively stable, with Republicans running ahead of Democrats in every decade save for the 1930 and 1970s, and usually by roughly 10 percentage points.

Figure 1 – Open seat hold rate by party

Simple bivariate relationships can sometimes be deceptive, however, obscuring other factors.  To see if the observed bivariate relationship is robust, we also estimated a model to account for other factors that might influence partisan hold rates. Our analysis focuses on voluntary departures from the US Senate, so we include both retirements from public life (of which there are 256 cases over our time series) and cases of progressive ambition (in which senators leave the chamber to pursue some other office, of which there are 58 cases); we do not include deaths in office, which only in the rare cases of suicide are voluntary. In addition to party, our model also controlled for a number of factors including measures of each state’s underlying partisanship, the presidency, Congressional control, as well as historical factors and the state’s population.

The results from our model indicate that – once other factors have been accounted for – Republicans are between 1 and 9.6 percent more likely to hold their open seats than Democrats.  That is not only itself statistically significant, but indicates that the roughly 10 percent advantage for Republicans seen in our initial analysis is still the case even after controlling for a number of variables.

While our historical data cannot speak directly or definitively to what will happen in 2018, they do seem to urge caution, since Republicans consistently tend to out preform Democrats in holding open Senate seats. Our analysis suggests that Republicans have organizational advantages that have contributed to a long-standing and on-going partisan advantage when it comes to retaining open seats.  To underscore the importance of our findings, it is worth referring back to the figure. Over the broad sweep of the time series, from the 66th to the 113th Congress, Republicans enjoyed a 10 point advantage in holding open Senate seats.  Over time, moreover, that advantage has grown.  Using the periodization developed by Sean Theriault and focusing on the most recent, “partisan” era, which Theriault dates from the 97th Congress (1981-1983), we  see that Republicans have retained 47 of 62 Senate openings (75.8 percent) contrasted to the Democrats’ 58.3 percent (42 of 72).

Understanding the dynamics of partisan holds of open Senate seats due to retirement provides important insights into the composition and control of Congress. It is not enough that Republicans have four times as many Senate retirements as do Democrats.  Democrats need to capitalize on these openings better than the data suggest they historically have.  If they cannot, Democratic hopes of regaining and retaining a Senate majority may be significantly more distant than they appear at first blush.

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Note:  This article gives the views of the author, and not the position of USAPP– American Politics and Policy, nor of the London School of Economics.  

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About the authors 

Hanna K. Brant – University of Missouri
Hanna K. Brant is a PhD student in the Department of Political Science at the University of Missouri. Her primary area of research is American politics, with an emphasis on Congress. She is currently interested in understanding how congressional staffers supplement congressional capacity to draft legislation and conduct oversight.

L. Marvin Overby – University of Missouri
L. Marvin Overby is a Professor in the Department of Political Science at the University of Missouri. His research and teaching interests are broad, and include both a number of topics in American politics as well as some in the area of comparative legislative analysis. 


Theodore J. Masthay – Wabash College
Theodore Masthay is a Visiting Assistant Professor of political science at Wabash College. His main area of study is American politics, with a focus on legislative careers. In particular, he is interested in understanding the ways in which the career trajectories of Democrats and Republicans differ.