President Barack Obama and Republican challenger Mitt Romney battled down to the wire on Tuesday, mounting a last-minute Election Day drive to get their supporters to the polls in a handful of states that will decide the winner in a neck-and-neck race for the White House.
Capping a long and bitter presidential campaign, Americans cast their votes at polling stations across the country.
At least 120 million people were expected to render judgment on whether to give Obama a second term or replace him with Romney.
Their decision will set the country's course for the next four years on spending, taxes, healthcare and foreign policy challenges like the rise of China and Iran's nuclear ambitions.
National opinion polls show Obama and Romney in a virtual dead heat, although the Democratic incumbent has a slight advantage in several vital swing states - most notably Ohio - that could give him the 270 electoral votes needed to win the state-by-state contest.
Romney, the multimillionaire former head of a private equity firm, would be the first Mormon president and one of the wealthiest Americans to occupy the White House.
Obama, the country's first black president, seeks to avoid being relegated to a single term, something that has happened to only one of the previous three occupants of the White House.
Whichever candidate wins, a razor-thin margin would not bode well for the clear mandate needed to help break the partisan gridlock in Washington.
Romney voted at a community center near his home in a Boston suburb, before dashing off for a pair of last-minute stops, including Ohio. "I feel great about Ohio," he said when asked about a state that is considered a must-win for him.
Underscoring the importance both campaigns have placed on Ohio, Vice President Joe Biden landed in Cleveland for a surprise visit just minutes after Romney touched down, in what looked like an attempt to steal the Republican's thunder.
Romney stayed onboard until Biden's motorcade cleared the tarmac, which soon became even more crowded when Romney running mate Paul Ryan's plane landed.
Settling into his hometown of Chicago, Obama made a final pitch to morning commuters in toss-up states that have been an almost obsessive focus of both campaigns, and made a surprise visit to a local field office staffed with volunteers.
"Four years ago, we had incredible turnout," Obama told a Miami radio station in a pre-recorded interview. "I know people were excited and energized about the prospect of making history, but we have to preserve the gains we've made."
He called a hip-hop music station in Tampa, Florida, in a final outreach to African-American supporters, saying that voting was "central to moving our community forward."
Fueled by record spending on negative ads, the battle between the two men was focused primarily on the lagging economic recovery and persistently high unemployment, but at times it also turned personal.
As Americans headed to voting booths, campaign teams for both candidates worked the phones feverishly to mobilize supporters to cast their ballots.
Polls will begin to close in Indiana and Kentucky at 6 p.m. EST (2300 GMT) on Tuesday, with voting ending across the country over the next six hours.
Ohio closes at 7:30 p.m. EST.
The first results, by tradition, were tallied in Dixville Notch and Hart's Location, both in New Hampshire, shortly after midnight (0500 GMT).
Obama and Romney each received five votes in Dixville Notch. In Hart's Location, Obama had 23 votes to nine for Romney and two for Libertarian candidate Gary Johnson.
The close presidential race raises the prospect of a disputed outcome similar to the 2000 election, which ended with a U.S. Supreme Court decision favouring George W. Bush over Al Gore after legal challenges to the close vote in Florida.
Both the Romney and Obama campaigns have assembled legal teams to deal with possible voting problems, challenges or recounts.
The balance of power in the U.S. Congress also will be at stake in Senate and House of Representatives races that could impact the outcome of "fiscal cliff" negotiations on spending cuts and tax increases, which kick in at the end of the year unless a deal is reached.
Obama's Democrats are now expected to narrowly hold their Senate majority, while Romney's Republicans are favoured to retain House control.
Amid uncertainty over the U.S. election outcome, no major moves were expected in global financial markets while investors waited for the result. World stock markets edged higher, and U.S. exchanges also rose on below-average volume.
Despite the weak economy, Obama appeared in September to be cruising to a relatively easy win after a strong party convention and a series of stumbles by Romney, including a secretly recorded video showing the Republican writing off 47 percent of the electorate as government-dependent victims.
But Romney rebounded in the first debate on Oct. 3 in Denver, where his sure-footed criticism of the president and Obama's listless response started a slow rise for Romney in polls.
Obama seemed to regain his footing in recent days at the head of federal relief efforts for victims of superstorm Sandy in the New York-New Jersey area.
The presidential contest is now likely to be determined by voter turnout - specifically, how many Republicans, Democrats and independent voters show up at polling stations.
Weather could be a factor. Much of the nation was dry and mild, though rain was forecast later on Tuesday in the Southeast, including Florida, an important swing state.
In the closing act of the 2012 election drama, both men expressed confidence in winning. But Obama hedged slightly, saying, after the Chicago campaign office visit, that "it's going to depend ultimately on whether those votes turn out."