Our new book is The Comeback: The 2024 Elections and American Politics. It includes a chapter on congressional and state elections.
Alan Abramowitz at The Center for Politics documents how House and Senate elections have become more nationalized and polarized. But significant seat swings are possible if the national vote margin is big enough.
The results in Table 5 suggest that a large Democratic seat swing is possible in both the House and Senate, although we have much more confidence in the House predictions. The national popular vote results will not be known until after the election, but we can use recent generic ballot polling, averaging the results of pre-election polls asking voters which party they would support in the congressional election, to estimate the likely outcome of the national popular vote. However, the generic ballot is a much more accurate predictor of the national House popular vote than of the national Senate popular vote since not all states hold Senate elections in a given year. The correlation between the generic ballot and the national House popular vote is .89 while the correlation between the generic ballot and the national Senate popular vote margin is a much weaker .75.
A Democratic vote margin of 5-6 points, roughly what recent generic ballot polls have been averaging, would result in an expected Democratic gain of about 20-25 seats in the House and 6-7 seats in the Senate, more than enough to shift control of both chambers. However, the group of states holding Senate elections in 2026 is considerably more Republican than the nation. In 2020, the last year in which this Senate class was up for election, Democrats won the national popular vote for the House by 3.1 percentage points but lost the national popular vote for the Senate by 2.3 percentage points. A 5-6 point popular vote margin in the House elections might predict a less than one-point popular vote margin in the Senate elections and a gain of only about four seats—barely enough to shift control of the upper chamber. These results suggest that the battle for control of the Senate is likely to go down to the wire even if Democrats easily win a majority of seats in the House. Once again, we see that the peculiar make-up of the Senate, with its vast overrepresentation of sparsely populated rural states, poses a serious potential obstacle to electoral accountability in the 21st century.