Former president Donald Trump has cast his ballot in Florida with wife Melania, saying he is “very confident” as Americans head to the polls in an unpredictable and razor-close US election.

Polling stations in eastern states including New York opened at 6am on Tuesday (US time) and hour by hour citizens in more states were able to vote, with Alaska and Hawaii the last to do so.

“It looks like Republicans have shown up in force,” Trump said.

The first results to come through, from a tiny town near the border with Canada, mirrored the stark divide among Americans.

The six residents of Dixville Notch, New Hampshire, were split down the middle, with three votes for Trump and three for Vice President Kamala Harris.

The resort town has a tradition dating back to 1960 of being the first in the country to complete in-person voting on the stroke of midnight.

But in an election where tensions have run high, the setting in Dixville Notch could not have been more congenial; voting took place in the living room of the Tillotson House, with cookies and coffee and a couple of friendly dogs.

Kamala Harris and Donald Trump each received three votes in the town of Dixville Notch. Photo: AAP

The first polls will close at 6pm (eastern time) in Indiana and Kentucky (about 10 am AEDT) and follow around the country. That’s when counting can begin.

Trump urged Republican voters to “stay in line” and not be intimated as Americans turned up at polling stations and faced long queues.

In his final election forecast, respected prognosticator Nate Silver had Trump and Harris on almost 50-50 based on 80,000 election simulations.

Harris won 50.015 per cent of simulations and Trump 49.6 per cent.

“When I say the odds in this year’s presidential race are about as close as you can possibly get to 50/50, I’m not exaggerating,” he wrote.

Pollsters say neither of the candidates has a meaningful statistical lead, in the battleground states — Pennsylvania, Michigan, Wisconsin, Georgia, North Carolina, Nevada and Arizona — amplifying the sky-high uncertainty as the race hurtles to the finish line.

Either Trump or Harris could still comfortably win if the polls are off and several of these states are won by one or the other candidate.

But legal and political analysts warn there is a high likelihood that it could be days, or even weeks, before the next president is known if exceedingly tight races become mired in ballot recounts and legal fights.

About 240 million people are eligible to vote in the election, which will also decide the make-up of the two houses of Congress: the House of Representatives and the Senate.

Which party controls those will have a major impact on the policy agenda of the incoming president.

Republicans have an easier path in the US Senate, where Democrats are defending several seats in Republican-leaning states, while the House of Representatives looks like a toss-up.

But election day is not what it used to be.

More than 75 million people have already cast their ballots by post or in-person early voting.

That includes current President Joe Biden, who voted in his home state of Delaware last month.

Harris has made abortion rights and cost-of-living concerns the centrepieces of her campaign. But the former prosecutor and senator also focused on the threat she says Trump — who was found guilty in a criminal hush-money trial in May — poses to democracy.

She called him a “petty tyrant” who is “out for unchecked power”.

Trump’s populist pitch to voters revolved around immigration, inflation and foreign policy, with promises of mass deportations, across-the-board tariffs, rolling back green energy initiatives and ending the war in Ukraine “in 24 hours”.

Harris and Trump spent the final weekend barnstorming the swing states in search of every available vote.

Trump staged his final rally on Monday night in Grand Rapids, Michigan, while Harris held twin rallies in Pittsburgh and Philadelphia.

Republican vice-presidential candidate JD Vance voted in Cincinnati on Tuesday.

“Look, I feel good. You never know until you know but I feel good about this race,” Vance said after he and his wife cast their ballots.

Vance said he would leave for Florida later on Tuesday to be with Trump as results came in.

If she succeeds, Harris — the daughter of an Indian-born mother and Jamaican-born father — would become the first female president in the history of the US.

A Trump victory would also be history-making: Only one other president — Grover Cleveland in the late 19th century — has succeeded in winning non-consecutive terms.

Another win for the 78-year-old real-estate tycoon, reality TV star and convicted felon would also make him the oldest person elected US president.

-with AAP