Key facts
- Warren, a Massachusetts Democrat, and Curtis, a Utah Republican, sent the letter to Kalshi's Tarek Mansour and Polymarket's Shayne Copeland.
- The senators wrote they were particularly troubled by election-related event contracts, which some observers treat as indicators of political momentum.
- The letter cited Kalshi's suspension of California gubernatorial candidate Kyle Langford after he bet on his own race in February 2026.
- Former Congressman George Santos faces a federal probe after allegedly betting on Kalshi whether he would attend Trump's State of the Union address.
- Kalshi enforcement head Robert DeNault said the platform prohibits insider trading and blocks candidates and campaign staff from trading.
- Polymarket did not respond to a request for comment.
A committee led by Senator Elizabeth Warren is probing Kalshi and Polymarket ahead of the midterm elections, according to a letter sent on October 28 to Kalshi CEO Tarek Mansour and Polymarket CEO Shayne Copeland. Warren, a Massachusetts Democrat, and Utah Republican John Curtis asked what both platforms are doing to ensure midterm odds have not been influenced unfairly.
The senators wrote that they were particularly troubled by election-related event contracts, which they said receive prominent coverage and are treated by some observers as indicators of political momentum. They raised concerns about users betting on elections with information that is not publicly available and about other trading behavior that could be considered market manipulation.
The letter challenged the platforms' position that the contracts users trade are financial products distinct from traditional gambling. It cited Kalshi's history, including a California gubernatorial race in February 2026 in which candidate Kyle Langford wagered on his own race. Kalshi suspended Langford. The senators also noted that former Congressman George Santos faces a federal probe after allegedly betting on Kalshi whether he would attend President Trump's State of the Union address.
Warren and Curtis highlighted similar instances involving Polymarket, including 2024 allegations of wash trading in election-related contracts, a practice of inflating trading volume by quickly buying and selling that is prohibited in other regulated markets. They also cited research finding that traders with inside information about political events, such as the timing of an Israeli strike against Iran, netted a collective $143 million on Polymarket between 2024 and 2026.
Kalshi has said it bans insider trading, a position it maintained after the Roosevelt Institute, a nonpartisan think tank, published an analysis claiming the platform gave professional traders an unfair advantage over regular users. Robert DeNault, head of enforcement at Kalshi, told Business Insider that the platform prohibits insider trading and market manipulation and proactively blocks candidates and campaign staff from trading to protect market integrity. Polymarket did not respond to a request for comment.