Fears of US civil unrest on election day have sparked security measures in battleground states that could decide the presidential election, as both candidates make their final pitches.

Donald Trump and his allies, who falsely claim his 2020 defeat was the result of fraud, have spent months laying the groundwork to again challenge the result if he loses.

Trump has promised “retribution” if elected, spoken of prosecuting his political rivals and described Democrats as the “enemy within”.

As the hours tick down to voting day on Tuesday (US time), nervous states have ramped up security measures.

National Guard troops have been called in to help maintain peace in at least three states, according to Reuters.

Arizona’s sheriff has placed drones and snipers on standby. In Las Vegas, Nevada, a security fence has been erected outside a building where votes are counted.

In Washington DC, the site of the 2021 riots, fences have gone up around government buildings such as the US Capitol and the Treasury Department.

Local businesses have boarded up windows in readiness for potential chaos on the streets.

The FBI has set up a special command post to monitor security threats.

It comes as the last full day of a presidential election unlike any other in US history was coming to a close.

Trump and Vice President Kamala Harris were both scrambling for an edge in a tight contest each portrays as an existential moment for America.

Both candidates converged on Pennsylvania on Monday (US time) to urge supporters who have not yet cast their ballots to show up on election day.

The state offers the largest share of votes in the Electoral College of any of the seven battleground states expected to determine the outcome.

In Pittsburgh, Trump appeared before a large crowd in an arena and offered what his campaign called his final closing message to voters.

“We’ve been waiting four years for this,” said Trump, who mounted a 2024 comeback bid after losing the 2020 election to Biden.

Trump pushed economic themes in his Pittsburgh speech, saying Harris would bring economic misery if elected.

“We’re going to win the commonwealth of Pennsylvania, and it’s going to be over,” said Trump, who later announced on stage he had been endorsed by podcaster Joe Rogan.

In Allentown, Harris predicted victory and promised to be a president for “all Americans”, as she appealed to the city’s substantial Puerto Rican community who were outraged by insults from a comedian at a Trump rally last week.

Later, Harris went doorknocking in Reading and held a brief rally in Pittsburgh, where pop star Katy Perry played a set.

Harris was scheduled to finish the day with a celebrity-filled event in Philadelphia, with appearances by Lady Gaga, Ricky Martin and Oprah Winfrey.

Oprah Winfrey and Vice President Kamala Harris at a campaign rally outside the Philadelphia Museum of Art, Pennsylvania. Photo: AAP

“Tomorrow is election day and the momentum is on our side,” Harris said in Pittsburgh.

“We know it is time for a new generation of leadership in America … And make no mistake, we will win,” she said to enthusiastic cheers.

Harris’s campaign team said its volunteers knocked on hundreds of thousands of doors in each of the battleground states at the weekend.

The campaign says its internal data shows that undecided voters are breaking in their favour, and says it has seen an increase in early voting among core parts of its coalition, including young voters and voters of colour.

Trump campaign officials said they were monitoring early-voting results that show more women have voted than men.

That is significant given that Harris led Trump by 50 per cent to 38 per cent among female registered voters, according to an October Reuters/Ipsos poll, while Trump led among men 48 per cent to 41 per cent.

“Men must vote!” the world’s richest person Elon Musk, a prominent Trump supporter, wrote on his X social media platform.

Trump’s campaign has outsourced most of the work to outside groups, including one run by tech billionaire Elon Musk, which have focused on contacting supporters who do not reliably take part in elections.

Trump said he was heartened by robust early-voting numbers in hurricane-ravaged part of North Carolina, which have leaned Republican in past elections.

One official said they thought Trump would carry North Carolina, Georgia and Arizona, which would still require him to carry one of battleground states in the Rust Belt – Michigan, Wisconsin or Pennsylvania – to win the White House.

Republicans also appeared to be posting strong early-vote results in Nevada.

Trump believes concerns about the economy and cost of living will carry him to the White House.

Trump will return to Palm Beach, Florida, to vote and await election results.

Harris is expected to spend election night at Howard University in Washington, a historically Black college that is her alma mater.

-with AAP