A race whipsawed by unprecedented events - two assassination attempts against Trump, President Joe Biden's surprise withdrawal and Harris' rapid rise - remained too close to call according to pollsters, even after billions of dollars in spending and months of frenetic campaigning.
The first ballots cast on election day mirrored the US divide.
Overnight, the six registered voters in the tiny hamlet of Dixville Notch, New Hampshire, split their votes between Harris and Trump in voting just past midnight.
Polling stations in eastern states including New York opened at 6am on Tuesday and hour by hour citizens in more states were able to vote, with Alaska and Hawaii the last to do so.
Early voting has already been up and running for weeks in many US states. (AP PHOTO)
Harris, the sitting Democratic vice president, and Trump, the Republican former president who is seeking a second four-year term, spent the final frenetic weeks of the campaign making their case to voters in the seven critical states that are likely to decide the winner.
Pollsters say neither of the candidates in the battleground states - Pennsylvania, Michigan, Wisconsin, Georgia, North Carolina, Nevada and Arizona - has a meaningful statistical lead, 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.
Donald Trump's vice presidential running mate JD Vance voted in Cincinnati with his family. (AP PHOTO)
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 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 to carry out mass deportations, impose across-the-board tariffs, roll back green energy initiatives and end 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 evening in Grand Rapids, Michigan, while Harris held twin rallies in Pittsburgh and Philadelphia.
Republican vice presidential candidate JD Vance voted in Cincinnati on Tuesday morning.
"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 depart for Palm Beach, Florida, later on Tuesday to be with Trump as results come 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 United States.
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 ever elected president.
with DPA and AP
