Midterm betting has exploded in scope: more than 21,000 markets and over $220M wagered this cycle

More contracts, more money

Betting on the 2026 US congressional midterms has expanded sharply on Kalshi and Polymarket. According to Reuters reporting published by Devdiscourse, the Washington-based Anti-Corruption Data Collective (ACDC) examined more than 21,000 markets through September 30 across Kalshi and Polymarket’s international and US platforms, compared with 463 markets covering the 2024 congressional races.

Trading volume across those platforms has exceeded $220 million this cycle, versus $92 million over the entire 2024 cycle, according to ACDC. The earlier total included wagers placed after Election Day while some races remained unresolved. The platforms gained prominence in 2024 after pointing toward a presidential victory for President Donald Trump despite contrary signals from traditional opinion polls.

Broader coverage, concentrated activity

The expansion reflects both coverage of more congressional seats and more contracts addressing specific aspects of each race. Winner contracts accounted for 71% of election markets in 2024 but represent only 16% this cycle. Contracts on candidates’ winning margins now make up 58%, and turnout markets have also become more numerous. Traders can additionally take binary yes-or-no positions on congressional control and whether candidates will withdraw.

That larger menu has not translated into evenly distributed trading. ACDC researcher Michelle Kendler-Kretsch said winner contracts still attract the overwhelming majority of volume, while many more specialized markets see little activity. Much of the $220 million is concentrated in a handful of contests, particularly the open Texas Senate seat contested by Democrat James Talarico and Republican Ken Paxton. Senate races in Maine, Michigan and South Carolina have also drawn substantial betting.

Activity is concentrated among traders, too. ACDC’s analysis of account, or wallet, data from Polymarket’s international platform found that the top 1% of wallets by congressional trading volume generated 70% of congressional betting volume this cycle. That finding applies to the international platform, rather than the full three-platform dataset.

Integrity questions accompany growth

Election contracts remain a small part of overall prediction-market trading, but some policymakers and public advocacy groups warn about insider trading and damage to confidence in US democracy. Kalshi has suspended a handful of candidates for wagering on their own contests and prohibits election trading by politicians and campaign workers. NPR reported in May that campaign staffers were betting on races they worked on. A Reuters/Ipsos poll published Monday found that most Americans oppose election wagering through prediction markets.

The platforms argue that their markets produce accurate, unbiased information and say they have safeguards against misconduct, routinely referring cases to US authorities. Kalshi describes its prices as information generated by participants putting money behind their views. A Polymarket spokesperson claimed a 98% accuracy rate for its election markets one day before an election and cited surveillance and detection systems designed for political trading.

What it means for prediction markets

• Market count is not a measure of trading depth. More than 21,000 contracts provide broader coverage, but many specialized markets remain lightly traded.
• Winner contracts still dominate participation. Margin markets account for 58% of listings, yet most volume continues to flow into bets on who wins.
• Concentration matters when interpreting prices. A few races attract much of the money, and the top 1% of wallets drive 70% of congressional volume on Polymarket’s international platform.
• Integrity remains a central issue. Candidate suspensions, reported campaign-staffer betting and public opposition put platform safeguards alongside forecasting performance as key questions for readers.