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(NEXSTAR) – For those who feel like there isn’t enough riding on the 2020 presidential election, there’s always gambling.

Bookies around the globe are taking lines on whether Donald Trump will pull another surprise upset on Election Day or whether former Vice President Joe Biden will close out the lead he’s maintained for much of the year. Strictly speaking, gambling on politics is not legal in the United States, so most numbers come from the United Kingdom or elsewhere.

For weeks the lines have given Biden much better odds than Trump to win the race but things shifted dramatically on the eve of the election, according to the gambling website OddsChecker.

“Yesterday, Joe Biden’s 2020 election odds implied he had a 65.2% chance of winning the 2020 election, that percentage has now dropped to 61.9% chance.” wrote Sam Eaton of OddsChecker.

So what happened? The shift could come from polling. Some final numbers show Trump drawing closer in Pennsylvania, which could be the decisive swing state. But there’s also another explanation.

It’s important to remember that bookies are trying to entice people to wager, not simply predict outcomes. Because of the way bets are structured, bookies guarantee the house wins money when there are nearly equal amounts bet on both sides, so if a heavy betting comes in on one side, the line may move to make the payout on that bet worse, as it did for Trump overnight.

As it turns out, OddsChecker reports that 71 percent of late bets Monday were for Mr. Trump and one major bet may have changed the way bookies view the contest. According to the Sun tabloid out of England, one British banker dropped a $5 million bet on the president in the closing days in what is being described as the largest ever bet on politics. The bet would reportedly pay nearly $15 million if it hits.

It’s not immediately clear whether that single bet, which was reportedly wagered in the Caribbean, was partially responsible for an overnight change in odds, but it’s unlikely that oddsmakers would want to see lopsided wagering on the president for fear of a massive round of payouts in the event of another upset.

As of Tuesday morning a $100 bet on Trump would win you $150 in the event of his reelection. You would have to bet $152 on Biden to win $100 from his victory, according to OddsChecker.