Most polls leading into Britain's June 23 referendum predicted Britons would choose to remain in the EU. Instead, they voted to leave by a 52 percent to 48 percent margin.
Betfair's "Next President" market was by far the largest of more than 70 markets on the site related to the U.S. election. As of Friday, some $140 million has been put into play on markets ranging from who will win the popular vote to how many states each party will carry.
On Ireland's Paddy Power, which merged with Betfair earlier this year, the U.S. presidential election "is definitely on course to be the biggest political event," said spokesman Féilim Mac An Iomaire. The site has had about $4.38 million bet on the race so far.
For Paddy Power, sporting events remain the site's "bread and butter," with politics more of a "niche market," Mac An Iomaire said. Nonetheless, he expects the 2016 race to be among the top 10-most-traded events on the site.
Even people with no knowledge of politics know who Trump is, said Mac An Iomaire said, explaining the runaway popularity of the U.S. election in betting markets. "Trump is such a huge celebrity."
Ladbrokes, a UK-based gambling company, said about 5 million pounds, or a little over $6 million, had been bet on the 2016 presidential election since its markets on the race opened four years ago. A Ladbrokes spokesman said that amount was "at least double" the amount wagered on the 2012 election.
The three sites all reported an 83 percent probability of a Clinton victory on Tuesday. Her probability shot up a few percentage points on Sunday after the FBI stood by its earlier recommendation that no criminal charges were warranted against Clinton in her use of a private email server.
Back in the USA
The vast majority of big-dollar political betting occurs outside America. Wagering on elections in the United States is limited by law to relatively small trading platforms connected to universities, which use data from the markets for academic research.
Still, the sites are abuzz with activity as the election nears, albeit in small amounts.
On PredictIt, launched in 2014 and jointly run by Victoria University in Wellington, New Zealand, and Washington, D.C.-based political consulting firm Aristotle International Inc, the five most-active markets all related to the presidential election.
PredictIt is not legally permitted to disclose the amount of money traded, but its top three markets have all reached the maximum number of participants.
Trading on the older Iowa Electronic Markets, launched by the University of Iowa in 1988, has also been brisk, although a spokeswoman said she could not say if 2016 was seeing substantially more volume than in 2012, when President Barack Obama won re-election against Republican Mitt Romney.
The latest Reuters/Ipsos poll showed Clinton with a 5 percentage point lead over Trump in the national survey — 44 percent to 39 percent — while races in the swing states of Florida, North Carolina had shifted from Clinton's control to being too close to call.
The PredictIt market gave Clinton an 81 percent probability of winning the White House. IEM's "Winner Takes All" market showed her with a 71 percent chance .
'She will win in a landslide'
Betfair said on Friday that Trump was emerging as the clear favorite among one group of players in particular - Brexit backers. Two-thirds of its customers who had supported a Brexit outcome were also backing Trump on Betfair.
Paul Krishnamurty, a professional gambler for 15 years who works as an analyst at Betfair and a columnist at Politico, is not one of them. He told Reuters he stood to lose $28,000 if Trump wins on Tuesday.
Krishnamurty, who said he had earned roughly half a million dollars during his betting career, has laid money on several election-related markets this year. He took a winning position on Trump's selection of Indiana Governor Mike Pence as his running mate and bet against Jeb Bush during the Republican nominating contests.
He said he believed Clinton would likely to win both North Carolina and Florida and that a Trump victory was unlikely.
"I personally believe she will win in a landslide," Krishnamurty said.