By Douglas Gillison and Michelle Price
WASHINGTON, Nov 3 (Reuters) - Prediction markets signaled growing expectations of a Democratic sweep of the US Congress on Tuesday, with more than $1.48 billion wagered on the midterm elections in a fresh test for the novel markets.
Betting prices moved further in favor of Democrats as polls began closing. On Tuesday around 7 p.m. ET (0000 GMT on Wednesday), prediction market prices aggregated by analytics firm BetInt implied Democrats had a 98% chance of taking
the House of Representatives, a four percentage point increase on mid-afternoon.
Amid high volatility, the markets at one point briefly implied an 88% chance of Democrats taking the Senate, a 14-percentage-point increase on earlier in the day, but retrenched to around 80%.
As the first polls closed at 6 p.m. ET (2300 GMT), roughly $2 million poured into bets that Democrats would take control of the Senate and House and prevail in hotly contested Senate races in Ohio, Texas and Maine and some gubernatorial contests, according to data provided by BetInt.
Prediction markets rose to prominence in 2024 when they bucked traditional opinion polls in signaling that Donald Trump would win the presidency, and have since amassed a global following among media outlets and investors.
The markets tout high accuracy rates in providing betting odds, but some high-profile misses during the US primaries and in Brazil's presidential election have shown they are fallible, and Tuesday's implied Democratic sweep will also be closely scrutinized.
If the midterm results diverge sharply from prediction-market odds, the industry’s reputation could suffer, analysts say, especially given the record sums being wagered.
Kalshi and Polymarket, the biggest platforms, are also under pressure to guard against misconduct after several candidates bet on their own races, and research from Washington nonprofit the Anti-Corruption Data Collective (ACDC) found political markets can be shifted with relatively small bets.
"Tuesday does, in fact, represent a real test for the new wave of prediction markets," said Yesha Yadav, associate dean at Vanderbilt University Law School, adding that this year's many congressional races could help contribute to the industry's overall scorecard.
Polymarket and Kalshi say they have ramped up controls and are well equipped to guard against misconduct. Kalshi blocks politicians and campaign workers from political wagers. The companies also say their markets quickly correct when large bets move prices.
Sean Patrick Maloney, CEO of the Coalition for Prediction Markets, said the markets do not pretend to be perfect but that in "a world full of chaos and biased news sources," they provide an unbiased way to forecast events.
STUDIES SAY MARKETS ARE WELL-CALIBRATED AT SCALE
Kalshi on Monday reported $1.33 billion in trading volume across all midterm bets, while about $145 million has been wagered on Polymarket's congressional markets, according to ACDC.
More than 28,000 midterm markets have been opened so far, with a fraction of individual contests, including the Senate races in Texas, Michigan, Maine and South Carolina, accounting for a large chunk of the trading volume, the nonprofit said.
The Texas Senate race, for example, had drawn nearly $67 million in wagers as of October 29, according to ACDC. On Tuesday evening, the markets implied a 75% chance that Texas Democratic candidate James Talarico will defeat the state's Republican Attorney General Ken Paxton for that seat, up 3 percentage points on earlier in the day, according to BetInt.
In the Michigan Senate race, which has drawn more than $26 million in wagers, the markets implied a 83% chance that Democrat Abdul El-Sayed will overcome Republican Mike Rogers, up 7 percentage points on earlier in the day, according to BetInt.
Academic studies, including a 2025 Vanderbilt University paper which analyzed nearly 2,500 markets on the 2024 election, have generally found prediction markets to be well calibrated overall. While calibration measures whether odds were reliable, accuracy measures whether the market picked the winner.
Because most races are not competitive, broad measures of performance do not necessarily show how well the markets perform in close contests, said Joshua Clinton, the Vanderbilt political science professor who led the 2025 research.
A small proportion of sophisticated profit-seeking traders appear to account for a large chunk of prediction market capital and help to correct prices, according to Harry Crane, professor of statistics at Rutgers University.
Still, even the most expert traders can get it wrong, Clinton said.
Prices on Kalshi and Polymarket implied just a 20% chance, for example, of Brazilian President Luiz Inacio Lula da Silva defeating his right-wing rival Flavio Bolsonaro, which he did last month. That may have been because sophisticated traders over-corrected for the risk that polls often underestimate the pull of right-wing candidates, said Clinton.
Company spokespeople said that events with a 20% chance of occurring happen all the time and political upsets are no different.
Carlos Hardenberg, portfolio manager at MCP Emerging Markets, said the lesson for investors was that the markets are a useful gauge. "But they should not be confused with certainty," he said.
(Additional reporting by Hannah Lang in New York and Johann M Cherian in Bengalaru; Editing by Jonathan Spicer and Deepa Babington)
