Election Day voting began in the United States on Tuesday after an extraordinary, and for many unnerving, presidential race that will either make Kamala Harris the first woman president in the country's history or hand Donald Trump a comeback that sends shockwaves around the world.

As the first polling stations opened, Democratic vice president Harris, 60, and Republican former president Trump, 78, were dead-even in the tightest and most volatile White House contest 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.

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 from 6.00am in states including Virginia, North Carolina and New York. 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.

Harris and Trump are effectively tied in the seven main swing states: Arizona, Georgia, Michigan, Nevada, North Carolina, Pennsylvania and 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. (AFP)