An Anti-Corruption Data Collective analysis found more than 21,000 midterm congressional markets on Kalshi and Polymarket through Sept. 30, up from 463 in 2024. Trading is concentrated in a few races and a small set of accounts.
Election betting is no longer a sideshow to U.S. politics. Three weeks before the midterms, it is a market with its own concentration risks.
More than $220 million has been traded on midterm congressional contracts this cycle, compared with $92 million for all of 2024, according to an Anti-Corruption Data Collective analysis of data from Kalshi and from Polymarket's U.S. and international platforms, cited in press reports Wednesday. The number of congressional markets has grown faster still: more than 21,000 through Sept. 30, against 463 two years ago.
What people are betting on
The menu has changed. About 58% of the markets are on margins of victory rather than who wins, and 16% are on winners. Most of the money, however, still goes to the winner markets.
Volume is concentrated in a handful of contests. The Texas Senate contest between Talarico and Paxton, along with races in Maine, Michigan and South Carolina, draws the most trading.
Who is betting
On Polymarket's international platform, the top 1% of wallets account for 70% of congressional trading volume. A market that thin at the top is one where a few large participants can move prices.
Kalshi has suspended a handful of candidates for betting on their own races. A Polymarket spokesperson said its markets had been 98% accurate a day before elections, a claim about the company's own performance.
The legal backdrop
The volume is building while Kalshi's legal footing is contested at the Supreme Court, where former Commodity Futures Trading Commission Chairman Gary Gensler and gambling-industry groups are among those filing briefs backing New Jersey's petition. Kalshi's response there is due Nov. 9, six days after the election, so the busiest weeks of election trading will pass before the court hears from the company.
Market implications
For the platforms, the volume figures are a growth story: election contracts have grown about 45-fold in market count and more than doubled in dollars in two years, with the most active weeks still ahead. For the broader market, the concentration figures are a caution. Odds that rest heavily on a small number of wallets may say more about those traders' positions than about voters.
What to watch
Volume in the final three weeks before Nov. 3, any further candidate suspensions or integrity actions by the platforms, and the Supreme Court's handling of New Jersey's petition in its case against Kalshi after the company responds.