The national tide falls short in the U.S. Senate

On the same night that Democrats celebrated a 40-seat pickup in the U.S. House of Representatives, they failed to gain control of the U.S. Senate. Faced with the highest number of seats a party has had to defend since 1938, Senate Democrats were largely on the defensive during the 2018 electoral cycle. Making matters worse, 10 Democratic senators were defending seats in states Donald Trump carried in the 2016 election. While Democrats gained two Republican-held seats by defeating Senator Dean Heller (R-NV) and winning retiring Sen. Jeff Flake’s (R-AZ) open seat, Republican Senate candidates successfully unseated Democratic Senators in Florida, North Dakota, Missouri, and Indiana.

While Democrats failed to win control of the Senate, Democratic senators in decidedly Republican states, such as West Virginia and Montana, were able to secure re-election. The figure below compares 2016 Democratic presidential performance to 2018 Democratic Senate performance and demonstrates the resiliency of these Senators. In fact, Democratic Senate candidates in 2018 out-ran Hillary Clinton’s 2016 performance in every state except for Utah, New Jersey, and Massachusetts.

For example, Senators Joe Manchin (D-WV) and Jon Tester (D-MT) were able to secure a 3-percent and 4-percent re-election victory in a state Hillary Clinton lost by 42-percent and 20-percent, respectively. Even in the conservative states where Democratic senators failed to win re-election (North Dakota, Indiana, Missouri), Democratic senators still out-paced their party’s 2016 presidential performance by over 13-percent. The most pronounced case of this was Senator Heidi Heitkamp (D-ND) who lost by 11-percent to U.S. Rep. Kevin Cramer (R-ND) while out-running Hillary Clinton’s 2016 performance by 25-percent. In all, Democrats lost 4 of the 10 incumbents running in states won by Donald Trump.

Increasingly partisan-centered U.S. Senate elections

 The case of the 2018 Senate elections underscores the recent partisan-centered nature of U.S. Senate elections. In a recent article published in Electoral Studies, I show that increasing partisan polarization in the Senate increases the salience of partisanship and incumbency in determining Senate election outcomes. Borrowing on previous theories of electoral choice, I argue that polarization increases the salience of partisan differences at the expense of candidate characteristics given that polarization further clarifies the link between individual candidates and their party. Increasingly responsible and ideological homogenous parties cement the link between local voting choice and national policy, to the determent of individual candidate brands.

Using data on all Senate elections during the direct-election era from 1920-2016, I show in the table below that greater polarization corresponds with a greater correlation between state partisan preferences and Senator party while also corresponding to a sharp decline in the proportion of out-partisan Senators from politically hostile states. (Note: Democratic (Republican) Senators from Republican (Democratic) leaning states are defined as Out-Partisan Senators representing politically hostile states.)

Given this descriptive finding, I specify a statistical model of Senate elections and find that polarization positively conditions the influence of partisanship and incumbency on Senate election outcomes. Further exploration finds that polarization positively conditions this incumbency advantage for Senators representing states that decidedly lean towards the opposing party and not for those representing marginal or partisan safe states. This finding, with respect to incumbency, is presented in the figure below. (Note that the incumbency advantage is defined as the electoral advantage an incumbent provides their party over a baseline non-incumbent candidate in an open-seat contest.)

Effect of incumbency on vote share by seat type and polarization level

Senators from out-partisan seats are privileged with a greater incumbency advantage as the Senate becomes more polarized. Akin to prize fighters adapting to higher levels of competition, out-partisan senators who survive electorally during periods of high partisan polarization are incumbents with the high level of political skill necessary to carve a distinct personal brand when partisan differences are clear. As a consequence, the utility of having out-partisan senators at the top of the ticket increases for parties tasked with defending seats in which they constitute a partisan minority as the Senate experiences higher levels of partisan polarization. I confirm this by using individual-level survey data on Senate elections and find that out-partisan senators are better equipped at garnering the support of partisan independents and opposing partisans as the Senate becomes more polarized.

Connecting the empirical findings and the 2018 Senate elections

My findings present some clear implications for partisan competition in Senate elections moving forward. First, as the Senate continues to polarize, the salience of partisanship will continue to grow. This results in greater partisan continuity in Senate seats, as parties can rely on this ever-growing partisan advantage to ensure victory in relatively uncompetitive contests, even during open seat races. As a consequence, this limits the number of Senate seats that can be considered potentially competitive between the two parties, even during pronounced partisan wave years such as 2018.

Second, while polarization positively conditions the incumbency advantage for out-partisan senators, out-parties holding Senate seats in which they are the partisan minority are living on borrowed time. Indeed, without the brand of incumbency attempting to offset a growing partisan disadvantage, these parties are assured losing their seat to the majority party. For example, the West Virginia Democratic Party is all but assured to surrender control of Senator Joe Manchin’s seat – a seat they’ve held since the election of Robert C. Byrd in 1958 – if Manchin retires in 2024.

The 2018 U.S. Senate elections also magnify an implicit finding presented in my research. While out-partisan senators do enjoy a greater incumbency advantage as the Senate becomes more polarized, this by no means guarantees re-election. Consider the case of Former Senator Heidi Heitkamp (D-ND). While Senator Heitkamp improved on her party’s 2016 performance by 25-percent, and assuredly came far closer to winning than a generic non-incumbent Democratic candidate would have, she was not able to overcome the increasing partisan advantage a Republican running for Senate has in a state as conservative as North Dakota. Indeed, it is hard to imagine a competitive Senate election in North Dakota as the parties continue to polarize.

As this research suggests, the battle for control of the United States Senate will be waged in an increasingly small number of competitive state battlegrounds.