Betting markets have narrowed significantly on the eve of Tuesday’s presidential election, eroding Donald Trump’s lead over Kamala Harris as Americans cast their vote.

The former president and his allies have touted the betting market forecasts as more accurate than traditional polling in recent weeks, as the top platforms put him way ahead of Harris.

As election day nears, however, Trump’s victory odds have faded, and Harris even retook the lead on one platform this weekend.

Betting markets have surged in popularity during this election campaign, with prominent apps like Polymarket and Kalshi surging up the app stores.

Many betting market forecasts for who was most likely to win the election also diverged from typical opinion polls. While the polls pointed to an incredibly close contest for the White House, a few weeks ago betting platforms put Trump clearly ahead.

But Trump’s chances have apparently dwindled. Polymarket put them at 58% on Monday, down from 67% last week; Kalshi put them at 53%, down from 65%.

Another platform priced Harris’s odds as greater than Trump’s for the first time in almost a month. The Democrat had a 53% chance of victory, according to PredictIt, which gave Trump a 51% chance.

Bets in these markets are bids on political futures contracts. Buying a contract – like the prospect of a Harris, or Trump, presidency – drives the price of that contract, or the perceived probability of it happening, higher.

Should you have turned to Polymarket on Monday, for example, and bet on Trump, you would receive $1 for every 58 cents you wagered if he wins the election. If you bet on Harris, on the same platform, on the same day, you would receive $1 for every 43 cents wagered if she wins.

A striking poll in Iowa that unexpectedly put Harris ahead of Trump sent shock waves through America’s poll-watchers this weekend. Betting market experts say participants consider a range of factors when placing a wager, including the results of such surveys.

Questions have been raised in recent weeks over activity in the betting markets. When it emerged that one man had wagered more than $30m on a Trump victory on Polymarket, both he and the platform stressed it was not an attempt to manipulate the market.

“My intent is just making money,” the man, who called himself Théo, told the Wall Street Journal, claiming he had “absolutely no political agenda”.