The rivals are running almost neck-and-neck in national polls
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.
Race to the White House
Obama50%
Romney47%
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.
US media view
Dana Milbank, writing in the Washington Post, has spotted something "new and unusual about Mr Romney". "In the waning days of the campaign, Romney was uplifting, optimistic and inspirational." According to Sam Stein in the Huffington Post, Monday's late-night rally in Iowa emphasised how President Barack Obama's "business-like" second White House run lacked the "hope narrative" of the first.Jim Rutenberg and Jeff Zeleny in the New York Times said the Romney campaign was banking on lower turnout for the president than in 2008. Mr Obama's team, it said, believed it had rebuilt its coalition of support "just enough to win". This CNN video rounded up 21 memorable moments from the gruelling race.
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.
“Start Quote
The two men do see two visions - two different mirages of a future America, shimmering hazily on the horizon. But the trouble is there really are two Americas existing now. And the gulf between them is getting wider.”
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.
US election - the essentials
Interactive: Predict the presidentGuide: Battleground states
Obama and Romney - key issues
Six Senate races to watch
Some analysts fear the election will not be decided on Tuesday night if the state's vote becomes mired in legal battles.
On Tuesday Mr Romney is to hit the campaign trail again with
events in Pittsburgh, Pennsylvania and Cleveland, Ohio, before holding
an election night rally in Boston.Mr Obama will hold his own election night rally at a convention centre in Chicago.
The BBC will be providing full online live results of the US presidential election on 6 November. More details here