Tens of millions of Americans have begun voting to decide whether to re-elect President Barack Obama or hand the job to Republican Mitt Romney.
Polling opened first in eastern states and a winner could be known by midnight. Turnout will be crucial.
The voting ends a hard-fought race that began nearly two years ago and has cost more than $2bn (£1.3bn).
Polls show the race is neck and neck, although the president holds a slender polling lead in crucial swing states.
National polls by Washington Post/ABC News and the Pew Research Centre both give Mr Obama a three-point edge over his rival.
As many as 30 million voters have already cast their ballots, with more than 30 states allowing either absentee voting or in-person early voting.
On the stroke of midnight, the first votes were cast and quickly counted in the tiny village of Dixville Notch in New Hampshire. They resulted in a tie with five votes each for Mr Obama and Mr Romney.
Polling stations will begin closing in eastern states at 19:00 EST (00:00 GMT).
'Work not done'
Mr Obama has already voted in his adopted hometown of Chicago, becoming the first sitting presidential candidate ever to vote early. Mr Romney is expected to cast his own ballot in Belmont, Massachusetts, later on Tuesday.
The election is decided by the electoral college. Each state is given a number of electoral votes in rough proportion to its population. The candidate who wins 270 electoral votes - by prevailing in the mostly winner-take-all state contests - becomes president.
Also on Tuesday's ballot are a handful of state governors, one third of the seats in the 100-member US Senate and all 435 seats in the House of Representatives.
Republicans are expected to keep control of the House, while Democrats were tipped to do the same in the Senate.
The presidential candidates spent Monday frantically criss-crossing the crucial battleground states including Ohio, Florida, Iowa and Virginia, making final appeals to voters. Their task: Push their own supporters to the polls while persuading the sliver of undecided voters to back them.
In speeches, Mr Romney kept up his attack on Mr Obama's record, reciting a litany of statistics he says illustrate the president has failed to lift the US economy out of the worst downturn since the Great Depression that followed the stock market crash of 1929.
"If you believe we can do better, if you believe America should be on a better course, if you're tired of being tired... then I ask you to vote for real change," Mr Romney told a rally in a Virginia suburb of the capital, Washington DC.
The president appeared at rallies with singer Bruce Springsteen and rapper Jay-Z. He acknowledged frustration with the still-lagging economy but told voters "our work is not done yet".
"We've come too far to turn back now," the president said in Ohio. "We've come too far to let our hearts grow faint... We'll finish what we started. We'll renew those ties that bind us together and reaffirm the spirit that makes the United States of America the greatest nation on Earth."
Legal battles feared
With observers anticipating a close race, both sides have readied teams of lawyers for possible legal fights, especially in the critical battleground state of Ohio.
Some analysts fear the election will not be decided on
Tuesday night if the state's vote becomes mired in legal battles. - BBC