U.S. President Donald Trump's chances of reelection gained sharply overnight in online betting markets as Americans head to the polls on Tuesday, according to Betfair Exchange.

Trump's odds of winning the election improved to 39% from 35% on the UK-based platform, while Democrat challenger Joe Biden saw his chances dip to 61% from 65%.

The election is on track to be by far the biggest betting event on record, with 300 million pounds ($389 million) bet with the platform so far, Betfair said. It accepts bets right up until the result is announced and sees bets with it to hit the 400 million pound mark, double that of 2016.

Betters on British exchange Smarkets give Trump a 38% winning chance.

Trump’s average odds on election day in 2016 sat at 5/1 (a 17% chance) — 20% lower than his odds now.

Biden has a substantial lead in national opinion polls, although the contest is slightly closer in battleground states likely to decide the race.

One person had placed a one million-pound ($1.3 million) bet on Biden, Betfair said on Monday, the biggest political bet of all time. If Biden wins, the player would bag 1.54 million pounds.

($1 = 0.7710 pounds)