Polymarket investigates six-figure Senate bets placed before Times/Siena polls, but onchain data casts doubt on an insider leak

A Polymarket trader's well-timed Senate bets have become a test of how prediction markets police themselves. The New York Times reported on November 6 that researchers at Vanderbilt University flagged bets placed shortly before two Times/Siena polls came out. Polymarket says it spotted the trades and is investigating. Kalshi declined to comment to the Times.

According to The Block, one account bet $191,277 that Republican Senator Pete Ricketts would win in Nebraska, mostly in the early hours of October 31, about an hour before a poll showed him six points ahead. The same account put $622,608 on a Democratic win in Michigan before a November 1 poll showed Abdul El-Sayed ahead by 11 points. It made more than $77,000 and more than $180,000 on the two races.

The Block's onchain review complicates the story. It linked the wallets to a trader with a history of big bets held to the result, including about $1.05 million on Zohran Mamdani in New York's 2025 mayoral race. One academic it consulted said the pattern looked like directional bets rather than a reaction to the polls; another said the simplest explanation was no foul play. Brett Goldstein of Vanderbilt said the activity still "requires further investigation."