Midterm Election Voting Day Arrives: A Look at the Probabilities for the House and Senate Across Three Major Prediction Markets

According to PolyBeats monitoring, with the U.S. midterm elections voting on November 3, the three major prediction markets—Polymarket, Kalshi, and Predict.fun—currently have a trading volume of approximately $178 million in markets related to the power structure of the Senate and House. The three platforms give highly consistent results: the Democrats winning both the Senate and the House is the most likely outcome according to the market.

The probability of a "Democratic sweep of both the Senate and House" is 75.5%, 75%, and 76% on the three platforms, respectively. Mainstream polls also favor the Democrats: the Silver Bulletin's national congressional ballot polling average as of November 2 shows the Democrats leading the Republicans by 8.9 percentage points; the Seldon forecast model gives the Democrats a 79% chance of winning the Senate and a 99% chance of winning the House.

The probability of "Republicans hold the Senate, Democrats win the House" is 18.5%, 20%, and 18% on the three platforms, respectively. This is the second most likely outcome in the eyes of the market. The Polling Forecast model points out that polls have overestimated the Democrats in 5 of the past 6 elections; if polls this time also overestimate the Democrats by 3.5 percentage points, the Senate will still be controlled by the Republicans.

The probability of "Republicans hold both the Senate and House" is 5.5%, 6.8%, and 5% on the three platforms, respectively; the probability of "Democrats win the Senate, Republicans hold the House" is 0.45%, 0.9%, and 0.7%. Current mainstream analysis believes the Democrats are most likely to gain a net 12 to 22 seats in the House, far exceeding the threshold of 3 seats needed to retake a majority. The three platforms price the probability of "Democrats retake the House" at 95%, 93.6%, and 96%, respectively.