President Barack Obama will be re-elected, according to projections from CNN and multiple other news organizations.
Obama and Romney ran dead even in final polls that hinted at a result rivaling some of the closest presidential elections in history, reflecting the deep political chasm in the country.
A heavy turnout was reported in much of the nation, and both campaigns expressed confidence that they would prevail in what was expected to be a long night awaiting results from the eight states still up for grabs that will determine the victor.
Around the country, voters formed long lines at polling places after record numbers participated in early balloting, indicating a strong turnout.
Don Palmer, secretary of the Virginia State Board of Elections, said the turnout this year may be stronger than in 2008, when Obama became the first Democrat to win the Southern state in 44 years.
Sporadic reports of irregularities included malfunctioning voting machines and other problems, including electoral hardships for some struggling to recover from the devastation of Superstorm Sandy in states in the country's northeast.
A judge in Philadelphia, a heavily Democratic city, ordered election officials to cover a mural of Obama at one school used as a polling location after Republicans complained the painting violated election laws.
Elsewhere in the city, GOP poll monitors were being escorted into precincts by sheriff's deputies after some observers had been denied access earlier in the day, said Tasha Jamerson, a spokeswoman for the district attorney's office.
In New Jersey, which permitted electronic balloting in the aftermath of last week's storm, the state chapter of the American Civil Liberties Union headed to court on Tuesday on behalf of voters who said their requests for an electronic ballot weren't being acknowledged.
Candidates usually take Election Day off, but both sides continued to make public appearances even as voting was under way.
Obama visited a local Democratic election center in the Chicago area, while Vice President Joe Biden made "an unannounced but long-scheduled" stop in the key battleground state of Ohio.
Romney and his running mate, Rep. Paul Ryan, also arrived separately in Ohio, with Romney's campaign plane and Air Force Two, which carries the vice president, crossing paths at Cleveland's airport.
Earlier Tuesday, Romney cast his ballot outside of Boston, while Obama previously became the first sitting president to vote early when he did so in Chicago last month.
No matter who wins the presidency, the White House and Congress will face fresh pressure to legislate a comprehensive deficit reduction deal that has been stymied so far by intransigence on the issue of tax reform, with Republicans refusing to consider any kind of tax increase while Obama and Democrats insist on at least the wealthy paying a higher income tax rate.
Despite the building drama toward Election Day in the campaign expected to cost $2.6 billion, much of the outcome already was known.
Only a handful of states were considered up for grabs and both candidates and their campaigns concluded an exhausting final sprint through them over the weekend and on Monday.
The barnstorming amounted to a montage of Americana electioneering, with Obama and Romney shouting themselves hoarse before boisterous crowds, joined by top surrogates and star power such as Bruce Springsteen singing for Obama and Kid Rock for Romney.
In their final speeches, the candidates and their running mates blended inspirational visions for a better future with well-honed attacks in hopes of ensuring their committed supporters actually cast ballots while trying to coax votes from anyone still undecided.
Obama briefly waxed nostalgic at his first event on Monday in Madison, Wisconsin, referring to Springsteen when he said: "I get to fly around with him on the last day that I will ever campaign, so that's not a bad way to end things."
He cited accomplishments of his first term, including ending the war in Iraq, winding down the war in Afghanistan, passing health insurance and Wall Street reforms, and ending the "Don't Ask, Don't Tell" policy that banned openly gay and lesbian personnel from the military.
"I know what real change looks like," the president said, referring to what he characterizes as Romney's false claim of being an agent of needed change. "You've got cause to believe me because you've seen me fight for it and you've seen me deliver it. You have seen the scars on me to prove it. You have seen the gray hair on my head to show you what it means to fight for change. And you've been there with me. And after all we've been through together we can't give up now because we've got more change to do."
Emotion overtook the president at the end of the day.
His eyes welled with tears as he thanked the people "who've given so much to this campaign over the years," during a stop in Des Moines, Iowa -- a place where his first campaign gained an early foothold in his first run for the White House.
"You took this campaign and made it of your own and you organized yourselves block by block, neighborhood by neighborhood, county by county, starting a movement that spread across the country," Obama said wiping away tears three times as he talked.
In North Carolina, first lady Michelle Obama exhorted voters to endure the expected long lines to vote on Tuesday, telling a Charlotte crowd: "Once you are in that line, do not get out. Don't get out. And the waits could be long. We need you to wait it out."