On Our Minds Midterms A Blue House, a Coin Flip Senate

On Our Minds Midterms: A Blue House, a Coin-Flip Senate

Jan Groen We introduce a framework to aggregate projected midterm election outcomes across different source while considering historical polling misses. Based on this SG model there currently is a narrow difference in probability between a unified and a divided Congress. Tanishka Kapoor Going forward we will publish weekly updates from the SG election model.

Ali KHAN The national environment looks like a wave year, but the Senate map forces Democrats to win in battleground red-leaning states. Prediction markets are currently favouring Democrats to flip four of the competitive races and win control over Senate, although the SG model with a broader range of inputs is less certain about this outcome. Energy prices, tariffs, data centres and AI are all key issues that will determine control for the Senate.

KEY NUMBERS FROM THE SG MODEL 2026 U.S Midterms As we approach Election Day on November 3, the race for control of Congress is entering its final stretch. All 435 House seats and 35 Senate seats are on the ballot, including special elections in Florida and Ohio. Republicans hold a 53–47 Senate majority. They are defending 22 of the seats up this year, while Democrats are defending 13. To win the Senate, Democrats need a net gain of four seats, because the Vice President breaks a 50–50 tie. Given a much slimmer 218-214 Republican advantage in the House, Democrats need to flip a net of just three House seats in the fall to take back the House. Source: SG Cross Asset Research/ Economics

Historically, midterms have acted as a check on the party that holds the White House. Since the World War II, the president’s party has lost House seats in all but two midterm (1998 and 2002). Some shifts have been large: Republicans gained more than 50 House seats in 1994 and 63 in 2010, and Democrats gained around 40 in 2018. The Senate follows a different pattern, because only a third of seats are contested and the result depends heavily on which states are on the map. In 2018, the president's party gained Senate seats even while losing the House.

Given that the very narrow Republican majority in the House provides ample opportunity for the Democrats to flip that chamber of Congress, the Senate races have become key to the outcome of this year’s midterm elections. This year's map leans toward states that have voted Republican in recent presidential elections, which gives Republicans a structural cushion in a challenging national environment. With cost-of-living concerns dominating voters’ views and several races tied within the margin of error, however, both chambers remain in play, with many political pundits predicting a blue wave. As of September 17, prediction markets and forecasters suggest a highly competitive Senate battle, with Kalshi implying roughly a 43% chance of Republican control, Polymarket about 42%, and the Cook Political Report rating the chamber a toss-up, while two national generic-ballot polls released this week showed notable Democratic momentum, including a New York Times/Siena College survey with Democrats ahead by 8 points and an Economist/YouGov poll showing a 12-point Democratic advantage.

Summarizing the Race Landscape: A Close Battle To summarize the wide range of expected outcomes for the Senate races in particular, we built a model that aggregates the views from analysts, surveys and prediction markets, while This document contains important disclaimer and disclosure information. Please refer to the back inside cover of this research report.

considering historical prediction errors relative to election outcomes from these sources. Within this framework we average expected outcomes from analysts, surveys and prediction markets, using relative weights that reflect the remaining time until the midterm elections (where polls are assumed to become more precise as we get closer to the elections) as well as quality and availability of the individual data sources. Additional prediction uncertain is built in by simulating prediction errors based in historical polling misses. Given the key role of the Senate races in determining the outcome of this year’s midterm…