Keeping It Classy: A Revised Model for Forecasting Seat Swing in Midterm Senate Elections

KEY POINTS FROM THIS ARTICLE

— The growing connection between presidential and Senate results has given Republicans a structural advantage in Senate elections.

— That advantage is particularly pronounced in Senate Class 2, the group of Senate seats being contested this year.

— Despite the collective Republican lean of the Senate seats being contested this year, the current average Democratic lead in generic ballot polling along with the large number of Republican seats up for election suggests that Democrats have a realistic chance of gaining at least four seats to take control of the upper chamber.

A class-conscious Senate forecasting model

The U.S. Senate was not intended to be a democratic body. Prior to the adoption of the 17th Amendment to the Constitution in 1913, Senators were chosen by state legislatures rather than voters. Even after the advent of direct popular election, differences in state populations ensured that states with small populations were vastly overrepresented in the upper chamber—an advantage that has increased substantially over time. Today, a senator from California, the most populous state, represents more than 65 times as many people as a senator from Wyoming, the least populous state. And, of course, residents of the District of Columbia, with a larger population than Wyoming or Vermont, have no representation in the Senate.

Enormous differences in state populations have important political consequences for modern Senate elections because of two recent trends affecting these contests. First, over time, the average state presidential vote margin has become increasingly Republican relative to the national popular vote margin because the least populous states are now more Republican than the most populous states in presidential elections. At the same time, the relationship between presidential and Senate election outcomes has become much stronger over time. As a result of these two trends, Senate elections have had an increasing Republican bias in recent years.

Figure 1: Trend in average state vs. national Democratic presidential vote margin

Note: Eras are defined as Eisenhower 1952-1958, Kennedy/Johnson 1960-1966, Nixon/Ford 1968-1974, Carter 1976-1978, Reagan-Bush1 1980-1990, Clinton 1992-1998, Bush2 2000-2006, Obama 2008-14, Trump/Biden 2016-24.

Source: Data compiled by Gary Jacobson and author

Figure 1 displays the trend in the average Democratic state vote margin compared with the national vote margin in presidential elections since the 1950s. During the Eisenhower presidential era, which covers the 1952 through 1958 elections, there was no difference between the average state Democratic margin and the national Democratic margin. During that era, there was no Republican bias in Senate elections. In every presidential era since then, however, there has been a Republican advantage due to greater Republican strength in less populous states. Moreover, that advantage has increased over time so that in the Obama and Trump-Biden eras, which include the 2008-2024 elections, the average state Democratic vote margin has been more than 6 points more Republican than the national Democratic vote margin.

The growing Republican advantage seen in Figure 1 has become much more important in recent years due to a second major trend in American politics—the increasing nationalization of congressional elections. The outcomes of both Senate and House elections are now much more closely connected to the outcomes of presidential elections. The trend for Senate elections is very clear in Figure 2. This figure displays the trend in the percentage of Senate outcomes that were consistent with the outcome of the presidential election in the state in the same year, or in the case of midterm elections, the outcome in the previous presidential election.

Figure 2: Trend in consistency of state presidential and Senate election results

Source: Data compiled by Gary Jacobson and author

From the Eisenhower through the Clinton eras, there was little or no relationship between the outcomes of presidential and Senate elections. During these years, the party losing the presidential election was almost as likely to win a Senate election as the party winning the presidential election. Since then, however, the relationship between presidential and Senate elections has become much stronger. During the Trump-Biden era, which includes the 2016-2024 elections, over 90% of Senate elections were won by the party winning the current or previous presidential election in the state. As a result, the Republican advantage in presidential elections in less populous states has made it increasingly difficult for Democrats to win a majority of seats in the Senate: the average Democratic share of Senate seats has fallen from 57% during the Eisenhower era, 64% during the Kennedy-Johnson era, and 59% during the Nixon-Ford era to 51% during the Clinton era, 52% during the Bush2 era, and only 48% during the Trump-Biden era.

Senate classes and Senate elections

Along with differences in state population, Senate elections differ from House elections in another important respect. Because of staggered six-year terms, only about one-third of Senate seats are contested every two years. The 50 states are divided into three classes with each class, made up of 33 or 34 states, electing its members every six years.[1] Moreover, the three Senate classes differ significantly in their political leanings, as the evidence in Figure 3 demonstrates.

Figure 3: Trend in average Democratic vote margin in states vs. nation by Senate class

Note: There were no Class 3 Senate elections during the Carter era (1976-78)

Source: Data compiled by Gary Jacobson and author

Figure 3 displays the trend in the average Democratic vote margin in states vs. the nation for the three Senate classes. Since at least the George W. Bush era (2000-2004), Senate Class 2 has included the most Republican-leaning states followed by Senate Class 3, with Senate Class 1 including the least Republican-leaning states based on presidential election results. During the Trump-Biden era (2016-2024), the Class 2 states have had an average Republican lean of more than 10 points compared with the nation while Class 3 states have had a Republican lean of about 8 points and Class 1 states have had a Republican lean of only about 3 points.[2]

In the modern era of highly nationalized Senate elections, the outcomes of Senate elections have closely reflected these differences in the Republican lean of Senate classes in presidential elections. During the Trump-Biden era, Democratic candidates have won 66% of Senate elections in Class 1 states compared with only 39% of Senate elections in Class 3 states and 36% of Senate elections in Class 2 states. It is the states in Senate Class 2, the most Republican-leaning class, that will be holding their Senate elections in 2026.

A revised forecasting model

Generic ballot polling can be used to forecast seat swing in midterm congressional elections with a relatively high level of accuracy when combined with two other predictors: exposure to risk based on the numbers of seats that each party has at stake in the election and the party of the incumbent president. However, forecasts of seat swing in the House based on the generic ballot are generally more accurate than forecasts of seat swing in the Senate.[3]

In this article, I test a revised model for forecasting Senate seat swing that adds one additional predictor: a dummy variable for the Senate class that is up for election that year. This is in addition to the three other predictors of Senate seat swing, which are used in the original model: the Democratic margin on the generic ballot prior to the election, the number of Democratic seats up for election, and a dummy variable for the party of the incumbent president. The estimates for the revised Senate model are displayed in Table 1. The dependent variable in this models is the predicted change in Democratic seats.

Table 1: Generic ballot forecasting model of midterm Senate seat swing

The results displayed in Table 1 show that the dummy variable for states in Class 2 has a significant negative effect on predicted seat swing in midterm Senate elections. All four predictors are highly statistically significant with generic ballot margin and seats at stake having the strongest effects. However, the results indicate after controlling for the other three predictors, Democrats can expect to gain almost three fewer seats in a Class 2 midterm like 2026 than in a non-Class 2 (Class 1 or Class 3) midterm. However, this penalty is offset by the relatively small number of Democratic seats at stake—they are only defending 13 out of the 35 seats on the ballot this November. This means that Democrats are less exposed to risk than Republicans, who have 22 seats at stake.

Table 2: Conditional forecasts of Senate seat swing in 2026 elections

Table 2 displays conditional forecasts of Senate seat swing in the 2026 midterm election based on the estimates in Table 1. I have included forecasts for average generic ballot margins ranging from +2 Democratic to +12 Democratic. As of late August, and based on averages from polling aggregators RealClearPolitics, FiftyPlusOne, and Silver Bulletin, the Democratic generic ballot margin is roughly 6-7 points. That large a generic ballot margin would predict a Democratic gain of just over five seats. Probability-wise, this model gives Democrats a better than 50% chance of controlling the Senate.

Notes

[1] For an explanation of the division of the Senate into classes and a list of the states included in each class, see https://ballotpedia.org/Classes_of_United_States_Senators.

[2] A few states that are not part of a class are sometimes included in the results because of special elections due to deaths or resignations. For instance, this year there are special elections in Florida and Ohio, meaning that there are 35 Senate elections this year instead of just 33. However, these have little impact on the overall class averages.

[3] See, for example https://centerforpolitics.org/crystalball/generic-ballot-model-gives-democrats-strong-chance-to-take-back-house-in-2026/. Most midterm forecasting models try to predict seat losses or gains for the party of the incumbent president. The model I am testing here predicts seat losses or gains for the Democratic Party and includes a dummy variable for the party of the president.

See Other Commentary by Dr. Alan Abramowitz.

See Other Political Commentary.

Views expressed in this column are those of the author, not those of Rasmussen Reports. Comments about this content should be directed to the author or syndicate.