Millions of Americans headed to vote Tuesday in a presidential election defined by drama and uncertainty, with Kamala Harris and Donald Trump left to await the outcome of a desperately close race after months of intense campaigning.The result -- perhaps coming overnight, or not for several days -- will either make Harris the first woman president in the country's history, or hand Trump a comeback that sends shock waves around the world.The bitter rivals spent their final campaign day trying to get supporters out to the polls and courting any last undecided voters in the swing states expected to decide the outcome.But Democratic vice president Harris, 60, and Republican former president Trump, 78, were in a dead-heat in opinion polls despite the most volatile White House contest of modern times.After head-spinning twists -- from Harris's dramatic entrance when President Joe Biden dropped out in July, to Trump riding out two assassination attempts and a criminal conviction -- tens of millions of voters are expected to cast their ballots, on top of the 83 million who have already voted early.Both candidates put in final pleas to voters Tuesday."Today, we vote because we love our country and we believe in the promise of America," Harris wrote on social media.Trump released a somber, one-minute ad, then posted: "This will be the most important day in American History."The Republican was expected to cast his vote later Tuesday in Florida, while Harris, who has already voted by mail-in, has radio interviews scheduled.Americans lined up across the country including in Black Mountain, North Carolina, where the Election Day voting station was a makeshift tent erected after severe flooding.Long queues also formed in Erie, a critical city in battleground Pennsylvania."It's way, way, way more people here than the last" election, Marchelle Beason, 46, told AFP after casting her ballot for Harris at an elementary school."We're so divided right now, and she's about peace. And everything that her opponent has to say is really negative," she added.At the same school, 56-year-old Darlene Taylor, who said she lives on disability benefits, noted her main issue is to "close the border.""We don't need another four more years of high inflation, gas prices (and) lying," said Taylor, who wore a homemade Trump shirt.Control of Congress, and by extension the ability for the new president to enact their agenda, is also at stake. All 435 seats in the House of Representatives are up for grabs, as are 34 of the Senate's 100 seats.A final presidential outcome may not be known for several days if the results are close, adding to tensions 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.Barriers have been erected around the White House, and extra security provided for election workers.The world is also anxiously watching, as the result will have major implications for conflicts in the Middle East, Russia's war in Ukraine, and tackling climate change, which Trump calls a hoax.On Monday, Harris went all-in on must-win Pennsylvania, rallying on the Philadelphia steps made famous in the "Rocky" movie and declaring: "Momentum is on our side."Trump -- who would become the first convicted felon and oldest person to win the presidency -- cast himself as the only solution to a country in terminal decline and overrun by "savage" migrants."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, Michigan.Harris hammered home her opposition to Trump-backed abortion bans in multiple states -- a vote-winning position with crucial women voters.At her final rally, she notably avoided mentioning Trump after targeting him directly as a threat to democracy for his dark rhetoric and repeated threats to exact retribution on his opponents.His return to power would instantly fuel international instability, with US allies in Europe and NATO alarmed by his isolationist "America First" policies.Trading partners are also nervously watching his vow to impose sweeping import tariffs.Trump has said he would not seek election again in 2028.But he also hinted he would refuse to accept another loss, and recently has brought up baseless claims of election fraud while saying he should "never have left" the White House.