Betting on US midterm elections has surged this year on Polymarket and Kalshi, prediction markets that rose to prominence in 2024 when they bucked traditional opinion polls in signaling that President Donald Trump would win the presidency.

The markets allow traders to buy and sell binary "yes" or "no" contracts on an array of race outcomes, such as which party will control Congress and which candidate will win individual races, as well as on questions such as when candidates may drop out. Although election bets remain a small slice of overall trading on prediction markets, some policymakers and public advocacy groups worry they could undermine faith in the integrity of US democracy and provide new avenues for insider trading.

Kalshi has suspended a handful of candidates for betting on their own races, while public broadcaster NPR reported in May that campaign staffers were also betting on the races they worked on. Polymarket, Kalshi and other platforms say their markets help reveal accurate, unbiased information.

They also say they are well equipped to guard against misconduct, and routinely refer cases to US authorities. Kalshi blocks election trades by politicians and campaign workers. But most Americans oppose using prediction markets to wager on elections, according to a Reuters/Ipsos opinion poll published Monday.

"Election prediction markets focus a disruptive new financial technology on our democratic process," said Michelle Kendler-Kretsch, a researcher at the Washington-based Anti-Corruption Data Collective (ACDC), which has researched betting patterns using data publicly disclosed by the platforms. "This data is bottom-up, not elites-driven," said a Kalshi spokesperson.

"Prediction markets are a way to see what people are saying when there's money on the line." Polymarket's election markets have a 98% accuracy rate one day before an election, a company spokesperson said.

"We’re providing reliable information to a public that is starved for it. We use state-of-the-art surveillance and dedicated detection systems built for political markets."

ACDC analyzed more than 21,000 markets on 2026 US congressional midterm elections across Kalshi and Polymarket's international and US platforms, through September 30, comparing them to 463 markets on the 2024 congressional races.

This cycle, the number of markets quickly eclipsed 2024 and continued to surge as the election neared. While more congressional seats are covered this cycle, the number of markets for each race has also grown, focusing on more detailed and precise race variables, according to ACDC.

Across the three markets, traders have bet more than $220 million so far this cycle, compared with $92 million for the entire 2024 cycle, according to ACDC. In 2024, wagers continued even after Election Day, as the outcome of some races hung in the balance. In 2024, 71% of election markets were on who would win a race.

This cycle, only 16% of markets relate to winners, with the majority – 58% – focused on the margin by which a candidate will win. There are also more markets this cycle on voter turnout for a race.

However, the vast majority of overall volume is still wagered on who will win, with many of the more esoteric election markets barely traded, according to Kendler-Kretsch. Overall, election bets are highly concentrated.

Most of the $220 million in trading volume is on a few races, most notably the Texas Senate race, in which Democratic candidate James Talarico faces Republican Ken Paxton for an open seat. Traders are also betting heavily on the Senate races in Maine, Michigan and South Carolina, ACDC said.

Data from Polymarket's international platform, which also provides data on accounts or "wallets," suggests a few traders dominate election bets.

The top 1% of wallets on Polymarket, ranked by trading volume on congressional markets, are responsible for 70% of all congressional betting volume this cycle, according to ACDC.