Betting markets fluctuated heavily overnight as the confidence in a sure presidential win for Democratic candidate Joe Biden ran into close contests on the ground across the U.S. Incumbent president Donald Trump started the day s a heavy underdog, but was heavily favored later in the night as he racked up unexpected wins and turned expected Democrat victories into contested states. As the mail-in ballots continue to be counted and the math turns back toward Biden, bettors have followed.

The British exchange markets saw its odds swing wildly, going from an 80% chance of a Trump victory last night to an 82% chance of a Biden victory today. The shift comes after Biden appears to have locked down a critical win in Wisconsin that gives him a path to 270 electoral votes required to win. 

"Taking the lead in Wisconsin could be the turning point with the Democrat now also projected to win Nevada and Arizona, which would likely give him the 270 electoral college votes he needs for victory," Betfair spokesperson Sam Rosbottom told Reuters.  

Betfair, for their part, is giving Biden a 78% chance of winning as of this morning. Ahead of the election, that platform saw its largest political bet ever: £1 million ($1.29 million) on a Biden victory. 

The whiplash caused a stir among gamblers, journalists and political pundits alike.