Ms Harris and Trump held competing rallies in Pittsburgh, Pennsylvania, last night in a final push for what some consider to be the most important of the swing states.

A late poll showed Ms Harris pulling ahead there, although most surveys agree the state and the election are tied.

Ms Harris kicked off election eve with a get-out-the-vote event in Scranton, Pennsylvania, urging supporters who were about to knock on doors for her to “enjoy” the final 24 hours of her campaign.

“Are you ready to do this?” she yelled, with a large handmade “VOTE FOR FREEDOM” sign behind her. She was set to hold a rally featuring performances by Katy Perry, and including remarks from a host of stars such as Lady Gaga, Ricky Martin and Oprah Winfrey.

US media, meanwhile, suggested Trump was also struggling to fill rallies, with cameras showing empty seats at early speeches.

At an early rally on Monday he called Puerto Rico “great” as he attempted to smooth over accusations of racism at a speech last week, and threatened 25 per cent tariffs on Mexico.

He also admitted he wanted to start attacking Michelle Obama at rallies after she called him “a small man trying to make himself big”, but was talked out of it by his advisers fearing that his message would land badly among female voters who have become critical.

Barriers around key sites in Washington, DC

Barriers had been erected around key sites in Washington, DC on Monday, including the Capitol, the White House and Kamala Harris’s residence.

Authorities are anticipating major protests across the country later in the week, which could be exacerbated if some states do not report final results for days.

In Washington, Oregon and Nevada, the National Guard has been placed on standby to deal with riots and civil unrest, which could begin as early as Wednesday.

Some election offices in Detroit and Atlanta, the largest cities in two key swing states, have been placed behind bulletproof glass to protect staff from rioting, while a ballot-counting warehouse in Philadelphia has been surrounded by barbed wire.

There were also suggestions on Monday that the Proud Boys, an extremist Right-wing group that played a key part in the riot on the Capitol in 2021, were planning to return to the streets after this election if Trump loses.