A final presidential outcome may not be known for 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 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.
'Momentum'
On Monday -- the last day of rallies -- Harris went all-in on must-win Pennsylvania, telling a huge crowd in Philadelphia: "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," he 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.
Trump's 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 anxiously watching his vow to impose sweeping import tariffs.