Nearly three-quarters of voters in the US presidential election say American democracy is under threat, according to preliminary national exit polls, reflecting the deep anxiety the nation faces after a contentious campaign between Democrat Kamala Harris and Republican Donald Trump.
The Edison Research results were released on Tuesday as the first of the nation's polls began to close.
The end of voting in parts of Indiana and Kentucky will be followed by the first large poll closing that will include most of Florida, all of Georgia and Virginia, among others.
Democracy and the economy rank by far as the most important issues for voters, with about a third of respondents citing each, followed by abortion and immigration at 14 per cent and 11 per cent, Edison's data shows.
The poll shows 73 per cent of voters believed democracy is in jeopardy, against just 25 per cent who say it's secure.
An Edison Research poll shows 73 per cent of voters believe democracy is in jeopardy. (AP PHOTO)
The data underscores the depth of polarisation in a nation whose divisions have only grown starker during a fiercely competitive race.
Trump has employed increasingly dark and apocalyptic rhetoric on the campaign trail, while stoking unfounded fears that the election system cannot be trusted.
Harris has urged Americans to come together, warning that a second Trump term would threaten the underpinnings of American democracy.
The figures represent just a slice of the tens of millions of people who have voted, both before and on election day on Tuesday, and the preliminary results are subject to change through the course of the night as more people are surveyed.
Forty-four per cent of voters viewed Trump favourably, compared with 46 per cent in the 2020 exit polls, when he lost to President Joe Biden.
Harris was viewed favourably by 48 per cent of respondents, compared with Biden's 52 per cent rating in 2020.
Donald Trump voted near his home in Florida, saying he would respect the result of a "fair" election (AP PHOTO)
Harris was relying on a large turnout by women voters to compensate for her electoral weakness with men.
The two rivals were hurtling toward an uncertain finish on Tuesday after a dizzying campaign as millions of American voters waited in calm, orderly lines to choose between two sharply different visions for the country.
A race churned by unprecedented events - two assassination attempts against Trump, Biden's surprise withdrawal and Harris' rapid rise - remained neck-and-neck after billions of dollars in spending and months of frenetic campaigning.
Trump, who has frequently spread false claims that he won the 2020 presidential election and whose supporters attacked the US Capitol on January 6, 2021, voted near his home in Palm Beach, Florida.
"If I lose an election, if it's a fair election, I'm gonna be the first one to acknowledge it," Trump told reporters.
Harris, who had earlier sent in her ballot by mail to her home state of California, spent some of Tuesday in radio interviews encouraging listeners to vote.
Kamala Harris spent election day doing radio interviews and imploring people to vote. (AP PHOTO)
Later, she was due to address students Howard University, a historically Black college in Washington where Harris was an undergraduate.
"To go back tonight to Howard University, my beloved alma mater, and be able to hopefully recognise this day for what it is is really full circle for me," Harris said on a radio interview.
Opinion polls before the election showed the candidates tied in each of the seven states likely to determine the winner: Arizona, Georgia, Michigan, Nevada, North Carolina, Pennsylvania and Wisconsin.
No matter who wins, history will be made.
Harris, 60, the first female vice president, would become the first woman, Black woman and South Asian American to win the presidency.
Trump, 78, the only president to be impeached twice and the first former president to be criminally convicted, would also become the first president to win non-consecutive terms in more than a century.
Voters have waited in calm, orderly lines to vote on sharply different visions for the US. (AP PHOTO)
Control of both chambers of Congress is also up for grabs.
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.
Tuesday's vote follows one of the most turbulent half-years in modern American politics.
In May, a New York jury found Trump guilty of falsifying business records to hide hush-money payments to a porn star.
Four weeks later, Trump and Biden met for their only debate, where the incumbent president delivered a disastrous performance that supercharged voters' existing concerns about the 81-year-old's mental acuity.
In July, Trump narrowly escaped a would-be assassin's bullet at a Pennsylvania rally.
Barely a week later, Biden exited the race.
with AP