If Donald Trump wins the U.S. election, it may not be Hillary Clinton and the Democrats who stand to lose the most.

After an unexpected rise in recent months, the just-confirmed Republican candidate is drawing unprecedented interest at bookmakers, setting the business up for huge losses if Trump emerges as the winner in November.

“The weight of money that we’re seeing on Trump has forced us to cut his odds. We’ve never had a presidential candidate so popular amongst bettors, and should he win it will be the worst political result for the bookmaking industry in history,” said Féilim Mac An Iomaire, a spokesman for Irish bookmaker Paddy Power IE:PPB.

The bookie recently shortened its odds on a Trump win to 15/8, giving him a 35% probability of moving into the White House. Back in April, Paddy Power put a 23% chance on a Trump win. More recently, the odds changed slightly to 2/1, or a 33% chance.

Read:Donald Trump is not the second coming of Ronald Reagan

“Normally the shorter priced candidate will attract the majority of the large bets, but there’s very little [difference] between him and Hillary in this sense as we’ve been seeing five-figure bets for Trump consistently since the turn of the year,” Mac An Iomaire said.

Rival bookie William Hill WMH, -1.87% has also seen an increase in bets on Trump in recent months and has shortened the odds on his winning significantly. When he entered the presidential race, he was only at 150/1, and that’s come all the way down to 7/4 this week.

Joe Crilly, press officer at William Hill, said the turnover for bets on this U.S. election in total is more than 300% times bigger than for the ballot in 2012. That’s partly because both Clinton and Trump are high-profile candidates, he explained.

“Trump becoming president would not quite be in the Leicester City league of upsets but in political betting it would certainly ‘trump’ the comparatively short upsets of the EU Referendum outcome and the Tories winning the General Election,” he said in a blog post.

Ladbrokes UK:LAD weren’t immediately available to comment. The odds of Trump winning were 7/4 on Thursday.

The bookies aren’t alone in re-assessing his presidential chances. Strategists at Citigroup said they now see a 35% probability of a Trump victory, up from around 20% before.

“ ‘In short, a Trump victory, while not our base case, cannot be ruled out.’ ” — Citigroup

“While political risks have historically been concentrated largely in EM countries, the combination of Brexit risk and the high degree of policy distance in the U.S. between the two presidential candidates highlights how political risks are now more prominent, and potentially destabilizing — even in developed democracies,” the strategists said.

“In short, a Trump victory, while not our base case, cannot be ruled out,” they added.

Meanwhile, as Trump gains more traction with voters, Clinton is seeing her popularity drop. Over the past few months, the Democrats’ candidate has gradually seen her lead narrow and polls now suggest a tight presidential race. According to an average of polls from Real Clear Politics, Clinton now stands to win 43.8% of the votes, compared with Trump’s 41.1%. That’s down from a 46.4%-to-39.6% split on June 27.

Read:Ben Carson links Hillary Clinton to Saul Alinsky and Lucifer at the Republican convention

This indicates Trump may have a greater chance at winning than the bookies currently give him credit for. Strategas Research Partners points out that the Clinton-Trump spread now is almost identical to the Brexit polls on referendum day, which gave the “remain” campaign a small, but convincing win.

Chart from Strategas Research Partners via Holger Zschaepitz.

And we all know how that went — 52% of the voters backed a call for the U.K. to leave the European Union, compared with the 48% who said they wanted to stay. That result was a huge shock to financial markets and bookies, which had put a more than 80% probability that Britain would vote to remain.

The surprise Brexit result could also be one of the factors that’s driving people to bet on Trump for U.S. president, said Paddy Power’s Mac An Iomaire.

“There’s been a string of surprises in politics on this side of the world of late (Conservative majority, Brexit, and to an extent May’s leadership win) and this is bound to influence bettors into thinking that there may be value in the underdog,” he said.