A presidential campaign marked by upheaval and rancor approached its finale on Election Day as Americans decided whether to send Donald Trump back to the White House or elevate Kamala Harris to the Oval Office.

Voters on Tuesday faced a stark choice between two candidates who have offered drastically different temperaments and visions for the world's largest economy and dominant military power.

Harris, the Democratic vice president, stands to be the first female president if elected. She has promised to work across the aisle to tackle economic worries and other issues without radically departing from the course set by President Joe Biden. Trump, the Republican former president, has vowed to replace thousands of federal workers with loyalists, impose sweeping tariffs on allies and foes alike, and stage the largest deportation operation in US history.

The two candidates spent the waning hours of the campaign overlapping in Pennsylvania, the biggest battleground state. They were trying to energise their bases as well as Americans still on the fence or debating whether to vote at all.

"It's important, it's my civic duty and it's important that I vote for myself and I vote for the democracy and the country which I supported for 22 years of my life,” said Ron Kessler, 54, an Air Force veteran from Pennsylvania who said he was voting for just the second time.

Harris and Trump entered Election Day focused on seven battleground states, five of them carried by Trump in 2016 before flipping to Biden in 2020: the “blue wall” of Pennsylvania, Michigan and Wisconsin as well as Arizona and Georgia. Nevada and North Carolina, which Democrats and Republicans respectively carried in the last two elections, also were closely contested.

The closeness of the race and the number of states in play raised the likelihood that once again a victor might not be known on election night. There was one early harbinger from the New Hampshire hamlet of Dixville Notch, which by tradition votes after midnight on Election Day. Dixville Notch split between Trump and Harris, with three votes for each.

Last time, it took four days to declare a winner. Regardless, Trump has baselessly claimed that if he lost, it would be due to fraud. Harris' campaign was preparing for him to try to declare victory before a winner is known on Tuesday night or to try to contest the result if she wins. Four years ago, Trump launched an effort to overturn the voters' will that ended in the January 6, 2021, insurrection at the US Capitol.

Trump planned to vote in his adopted home state of Florida on Tuesday, then spend the day at his Mar-a-Lago estate in advance of a party at a nearby convention centre.

Harris already voted by mail in her home state of California. She'll have a watch party at her alma mater, Howard University in Washington.

Each candidate would take the country into new terrain

Harris, 60, would be the first woman, Black woman and person of South Asian descent to serve as president. She also would be the first sitting vice president to win the White House in 32 years.

A victory would cap a whirlwind campaign unlike any other in American history. Harris ascended to the top of the Democratic ticket less than four months ago after Biden, facing massive pressure from his party after a disastrous debate performance, ended his reelection bid.

Trump, 78, would be the oldest president ever elected. He would also be the first defeated president in 132 years to win another term in the White House, and the first person convicted of a felony to take over the Oval Office.

Having left Washington abandoned by some allies after January 6, Trump defeated younger rivals in the Republican primary and consolidated the support of longtime allies and harsh critics within his party. He survived one assassination attempt by millimeters at a July rally. Secret Service agents foiled a second attempt in September.

A victory for Trump would affirm that enough voters put aside warnings from many of Trump's former aides or instead prioritised concerns about Biden and Harris' stewardship of the economy or the US-Mexico border.

It would all but ensure he avoids going to prison after being found guilty of his role in hiding hush-money payments to an adult film actress during his first run for president in 2016. His sentencing in that case could occur later this month. And upon taking office, Trump could end the federal investigation into his effort to overturn the 2020 election results.

The election has huge stakes for America and the world

The potential turbulence of a second Trump term has been magnified by his embrace of the Republican Party's far right and his disregard for long-held democratic norms.

Trump has used harsh rhetoric against Harris and other Democrats, calling them “demonic”, and has suggested military action against people he calls “enemies from within”.

Harris, pointing to the warnings of Trump's former aides, has labelled him a “fascist” and blamed Trump for putting women's lives in danger by nominating three of the justices who overturned Roe v. Wade. In the closing hours of the campaign, she tried to strike a more positive tone and went the entire last day Monday without saying her Republican opponent's name.

Heading into Election Day, federal, state and local officials expressed confidence in the integrity of the nation's election systems. They nonetheless were braced to contend with what they say is an unprecedented level of foreign disinformation — particularly from Russia and Iran — as well as the possibility of physical violence or cyberattacks.

Both sides have armies of lawyers in anticipation of legal challenges on and after Election Day. And law enforcement agencies nationwide are on high alert for potential violence.

The outcome of the race was being closely watched around the world, with the future of American support for Ukraine, US fidelity to its global alliances and the nation's commitment to stand up to autocrats hanging in the balance.

Harris has vowed to continue backing Kyiv's defence against Russia's full-scale 2022 invasion. Trump has sharply criticised Ukraine, praised Russian President Vladimir Putin and suggested he would encourage Russia to attack NATO allies of the US that Trump considers delinquent.

Voters nationwide also were deciding thousands of other races that will decide everything from control of Congress to state ballot measures on abortion access.

More than 81 million people voted early — shy of the record set during the 2020 pandemic, when Trump encouraged Republicans to stick to voting on Election Day. This time, he urged his voters to lock down their ballots in advance and they complied in droves.