Millions of dollars have poured into bets on who will win the US presidential election after a last-minute court ruling opened up gambling on the vote, upping the stakes on a too-close-to-call race between US Vice President Kamala Harris and former US president Donald Trump that has already put voters on edge.

Contracts for a Harris victory were trading between 48 and 50 percent in favor of the Democrat on Friday on Interactive Brokers, a firm that has taken advantage of a legal opening created earlier this month in the country’s long running regulatory battle over election markets.

With just a month until the Nov. 5 vote, markets opened after a court in Washington ruled that Kalshi, a start-up that has been trying to introduce political betting in the US for years, could take wagers as legal appeals by regulators against the company continue. Within days, more than US$6.3 million had been put on the line for the Harris-Trump matchup alone, with users also betting on control of the US House of Representatives and US Senate.

It is the latest turn in a years-long saga between the US Commodity Futures Trading Commission (CFTC) and firms wishing to offer election betting — a legal practice in a number of other countries and one that some Americans participate in outside of regulators’ oversight through offshore markets.

More than US$1.7 billion has been placed on the Harris-Trump matchup on one such offshore site, Polymarket, where Trump held a 54 to 45 advantage over Harris on Friday evening.

Those in favor of gambling — or “event contracts” in finance terms — say it is a legitimate way to hedge bets against adverse outcomes, likening it to futures contracts. Some also say that the markets are better than polls.

“These contracts are important,” said Steve Sanders, executive vice president of marketing and product development at Interactive Brokers. “They’re good for people to take a view on what they think is happening and hedge their portfolios.”

In just days, more than 1 million contracts had been traded on Interactive Brokers.

Only two election betting markets were legally operating in the US before the ruling on Wednesday last week, having been granted exemptions due to their affiliation with research projects and the strict limits they placed on the amount people could bet.

Critics worry about widespread election gambling in a polarized moment when basic facts are in dispute and disinformation on which people might base their wagers is abundant.

“I don’t want to be too dramatic, but we live in a country where tens of millions of Americans believe the last presidential election was stolen,” CFTC General Counsel Rob Schwartz said during recent arguments against Kalshi.

“Ensuring the integrity of elections and avoiding improper interference and misinformation are undoubtedly paramount public interests,” Judge Patricia Millett wrote in the decision that allowed bets to be placed while the appeals against Kalshi play out.

However, the CFTC had “given this court no concrete basis to conclude that event contracts would likely be a vehicle for such harms,” she added.

While the door on political betting might shut again on further appeal, any decision on that would likely come after Nov. 5, meaning markets would stay open through the election.

For Pratik Chougule, cofounder of the advocacy group Coalition for Political Forecasting, “it’s been a good year” so far for his own political wagers.

He said he is taking a look at opening an account with Interactive Brokers now that it is operating election contracts.

Judges on further appeals might be “a bit more skeptical,” but Chougule said that in the past few years the CFTC has warmed up to at least understanding the arguments in favor election markets, even if regulators do not endorse them.