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, who is also the former governor of Massachusetts, would be the first Mormon president and one of the wealthiest Americans to assume the nation's highest office.
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 four occupants of the White House.
Whichever candidate wins, a razor-thin margin might 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, which is considered a must-win. "People in Ohio know that they're probably going to decide who the next president is going to be," he told a radio station in the state.
In an awkward convergence of campaign planes that underscored the importance both sides have pinned on Ohio, Vice President Joe Biden made a surprise landing in Cleveland just minutes after Romney touched down, in what looked like an attempt to steal the Republican's thunder.
Romney stayed on board until the Biden motorcade cleared the tarmac, which soon became even more crowded when his Republican running mate, Paul Ryan, landed to join the Cleveland visit.
Settling into his hometown of Chicago, Obama delivered 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, telling listeners 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 following 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 race raises the prospect of a disputed outcome similar to the 2000 election, which ended with a U.S. Supreme Court decision favoring 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.
Complaints about voting procedures and possible irregularities surfaced sporadically across the electoral map, with frustrations running high in storm-battered New York and New Jersey.
But there was no immediate claim of anything widespread or systematic enough to cast doubt on the credibility of the election outcome.
The balance of power in the U.S. Congress also will be at stake in races for the Senate and
House of Representatives that could affect 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 favored to retain House control.
Despite uncertainty about the outcome of the presidential election, U.S. stocks climbed on speculation that it would produce a clear winner. World stock exchanges also rose, but the election kept trading subdued. "It's a relief that hopefully the election will be over," said Fred Dickson, chief market strategist at D.A. Davidson & Co. in Lake Oswego, Oregon.
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 presidential debate on October 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 delivered much stronger performances in two subsequent debates and has been praised in recent days for taking the lead in 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, and 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."
Like the divided electorate itself, Eoin O'Shea and his wife, Ann Marie, a South Philadelphia couple, split their vote. Both are 39 years old. He went for Obama. She supported Romney.
"We still love and respect each other," he said.
But some voters saw the choice in starker terms.
"It's about worrying about what is going to happen if our country is led by the wrong person," said Obama supporter Sylvia Zaal, 38, after voting in Milwaukee.
Voting appeared to go smoothly in most places.
But thousands of voters in New York and New Jersey encountered confusion and frustratingly long lines. Polling stations there were among thousands of buildings damaged by superstorm Sandy eight days ago.
Obama and Romney raced through seven battleground states on Monday, courting the last remaining undecided voters.
Obama focused on Wisconsin, Ohio and Iowa, swing states that, barring surprises elsewhere, would ensure he reaches the 270 electoral votes needed to win. Romney visited Florida, Virginia and Ohio before finishing in New Hampshire.
The president wrapped up his tour in Des Moines, Iowa, on Monday with a speech that hearkened back to how the state helped launch his candidacy of "hope and change" in 2008. He wiped away tears as he reflected on his political journey.
Romney ended Monday in Manchester, New Hampshire, where he started his campaign last year. "We're one day away from a fresh start," he told a rally.
Obama ridiculed Romney's claims to be the candidate of change and said the challenger would be a rubber stamp for a conservative Tea Party agenda. Romney argued that four more years of Obama could mean another economic recession.
The common denominator for both was Ohio. Without its 18 electoral votes, the path to victory is very narrow for Romney. No Republican has won the White House without winning the state.
Polls have shown Obama with a small but steady lead in Ohio, due in part by his support for a federal bailout of the auto industry, which accounts for one of every eight jobs there. That undercut Romney's central argument that his business experience made him uniquely qualified to create jobs.
Romney's aides also hoped an 11th-hour visit on Tuesday could boost his chances in Pennsylvania, a Democratic-leaning state that he has tried to put in play in recent weeks.
Obama fought back through the summer with ads criticizing Romney's experience at the private equity firm Bain Capital and portraying him as out of touch with ordinary Americans.
That was part of a barrage of advertising from both candidates and their party allies, who raised a combined $2 billion. The rise of "Super PACs" - unaffiliated outside groups - also helped fuel the record campaign spending.
Obama kept to his Election Day ritual of playing basketball with friends and staffers. He voted in October using early voting procedures - as have up to 35 to 40 percent of voters nationwide, either by mail-in ballots or in person.
Biden stood patiently to cast his ballot in his home state of Delaware. Asked if this would be the last time he would vote for himself, Biden he said with a grin: "No, I don't think so." The 69-year-old former U.S. senator, who twice ran unsuccessfully for the White House, has not ruled out a 2016 run.