Millions of dollars have poured into bets on who will win the US election after a last-minute court ruling opened up gambling on the vote, upping the stakes on a too-close-to-call race that has already put voters on edge.

Contracts for a Harris victory were trading between 48 and 50 per cent in favour 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 November 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 United States 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 match-up alone, with users also betting on control of the House and Senate.

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

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