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American voters deliver their verdict Tuesday after an extraordinarily turbulent election that will either make Kamala Harris the first woman president in US history or deliver Donald Trump a comeback that sends shockwaves worldwide.

As the first polling stations opened on Election Day, Democratic Vice President Harris, 60, and Republican former President Trump, 78, are dead even in the tightest and most volatile White House race of modern times.

The bitter rivals spent their final day of the campaign frenetically working to get their supporters out to the polls and trying to win over any last undecided voters in the swing states expected to decide the outcome.

But despite a series of head-spinning twists in the campaign -- from Harris's dramatic entrance when President Joe Biden dropped out in July, to Trump riding out two assassination attempts and a criminal conviction -- nothing has broken the deadlock in the opinion polls.

Polling stations opened at 6:00 am (1100 GMT) on the US east coast and tens of millions of voters are expected to cast their ballots, on top of the more than 82 million people who have already voted early in the preceding weeks.

A final outcome may not be known for several days if the results are as close as the polls suggest, adding to the tension in a deeply divided nation.

And there are fears of turmoil and even violence if Trump loses, and then contests the result as he did in 2020, with barriers erected around the White House and businesses boarded up in Washington.

The world is anxiously watching, as the outcome will have major implications for conflicts in the Middle East, for Russia's war in Ukraine, and for tackling climate change -- which Trump calls a hoax.

'Every single vote matters'

Harris and Trump are effectively tied in the seven main swing states -- Arizona, Georgia, Michigan, Nevada, North Carolina, Pennsylvania, Wisconsin.

On the eve of the vote, Harris went all-in on the must-win state of Pennsylvania, rallying on the Philadelphia steps made famous in the "Rocky" movie and declaring: "momentum is on our side."

However "this could be one of the closest races in history -- every single vote matters," cautioned Harris, who was joined by celebrities including Lady Gaga and Oprah Winfrey.

Trump -- who would become the first convicted felon and oldest person to win the presidency -- cast himself as the only solution to an apocalyptic vision of the country in terminal decline and overrun by "savage" migrants.

"With your vote tomorrow, we can fix every single problem our country faces and lead America -- indeed, the world -- to new heights of glory," Trump told his closing rally in Grand Rapids in the key swing state of Michigan, after touring North Carolina and Pennsylvania.

Harris meanwhile hammered home her opposition to Trump-backed abortion bans across the United States -- one of her key vote-winning positions with crucial women voters.

But she also struck an upbeat note -- and notably avoided mentioning Trump, after weeks of targeting him directly as a threat to democracy for his dark rhetoric and repeated threats to exact retribution on his political opponents.

History made

A Trump comeback would be historic -- just the second ever non-consecutive second term for a US president, since Grover Cleveland in 1893.

Trump's return would also instantly fuel international instability, with US allies in Europe and NATO alarmed by his isolationist "America First" policies. Trading partners are nervously watching his vow to impose sweeping import tariffs.

A Harris victory would give the US its first Black woman and South Asian president -- and signal an end to the Trump era which has dominated US politics for nearly a decade.

Trump has said he would not seek election again in 2028.

However, the Republican still refuses to admit he fairly lost the 2020 election to Biden, and the trauma over his supporters' violent attack on the US Capitol to stop certification of the result remains heavy.

Trump has hinted that he would refuse to accept another loss, and in the final days of the campaign brought up baseless claims of election fraud while saying he should "never have left" the White House.

© Agence France-Presse By Danny KEMP