Senate is still a tossup, but Democrats now have slight edge, prediction market traders think

Aaron Schwartz | Bloomberg | Getty Images

The battle for the U.S. Senate is still tight, but prediction market traders think Democrats have one of their best chances yet of taking the upper chamber.

Speculators on Kalshi now give Democrats a 54% chance to win the Senate, nearly matching a level reached in mid-April. On Polymarket, the odds are even higher, with a 59% chance that Democrats wrest control.

Republicans are defending majorities in both the U.S. House of Representatives and Senate this November, but the upper chamber was always viewed as more difficult for Democrats to flip. Republicans already control 20 of the 33 Senate seats up for grabs this year, and Democrats would have to flip several states where President Donald Trump won by 10% or more in 2024 — including Alaska, Texas and Ohio — to take control.

Odds that the Democrats win the Senate have improved significantly in 2026 on prediction markets. Before the U.S.-Iran war began on Feb. 28, Republicans had about 60% odds to hold onto the Senate on both Kalshi and Polymarket. Odds declined as rising gas prices drove down Trump's approval rating.

Odds that Democrats would win the Senate topped Republican odds of keeping control in April, but the GOP's chances recovered in May and throughout the summer as the U.S. and Iran deescalated the war, easing pressure on gas prices.

GOP fortunes have darkened in recent weeks. U.S. oil prices are now above $100 per barrel, gasoline is above $4 a gallon nationally and diesel prices are at an all-time high. Prediction market traders now think gas prices will hit new highs this year.

This week has brought several high-quality polls showing Democrats with a favorable environment heading into November. A New York Times/Siena University poll released Wednesday showed likely voters nationwide favoring Democratic candidates over Republicans in their congressional districts by close to 9 percentage points.

Disclosure: CNBC and Kalshi have a commercial relationship that includes customer acquisition and a minority investment.