From “Keeping It Classy: A Revised Model for Forecasting Seat Swing in Midterm Senate Elections” by Alan I. Abramowitz at centerforpolitics.org:
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.
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.




