The campaign has seen head-spinning twists: two assassination attempts and a felony conviction for Republican former president Trump, and the surprise elevation of Democratic Vice President Harris to the top of the ticket.
The latter came after President Joe Biden, 81, dropped his re-election bid under pressure from his own party. More than $US2.6 billion ($A4 billion) has been spent to sway voters' minds since March, according to analytics firm AdImpact.
Nevertheless, opinion polls show Trump, 78, and Harris, 60, virtually even. The winner may not be known for days after Tuesday's vote, though Trump has already signalled he will attempt to fight any defeat, as he did in 2020.
Republican presidential nominee Donald Trump pushed economic themes in a speech on election eve. (AP PHOTO)
Both candidates converged on Pennsylvania on Monday 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 in the last hours before election day.
"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.
Democratic nominee Kamala Harris said it is time for a new generation of leadership in America. (AP PHOTO)
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.
"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' campaign team said its volunteers knocked on hundreds of thousands of doors in each of the battleground states this 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.
Election campaigners and analysts note high enthusiasm and more early voters among women. (AP PHOTO)
Tom Bonier, head of the Democratic analytics firm TargetSmart, said the early vote showed high enthusiasm among Democratic-leaning groups, especially women. He said there was no indication of a similar surge among young men, a key target of the Trump campaign's outreach.
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 voter outreach work to outside groups, including one run by Musk, which have focused on contacting supporters who do not reliably participate in elections, rather than undecided voters.
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