"We won't make that mistake again," said senior adviser Robert Gibbs.
Romney was voting at a community center near his home in Belmont, Mass., before his sprint to Ohio and Pennsylvania. His campaign released a gauzy 5-minute Election Day web video called "The Moment" replaying key events from the campaign, with Romney assuring voters, "The future is better than the past."
The election played out with intensity in the small subset of battleground states: Colorado, Iowa, Florida, New Hampshire, Nevada, North Carolina, Ohio, Virginia and Wisconsin. Romney's late move to add Pennsylvania to the mix was an effort to expand his options, and Republicans poured millions into previously empty airwaves there.
In the campaign's final hours, voters around the country echoed the closing arguments of the two presidential candidates.
Obama supporter Gary Muratore, of Upper Arlington, Ohio, said Obama had rescued the country "from the brink of economic disaster."
"And while I don't think the pace of the recovery has been as fast as anyone would like, I think that the only way forward is to keep on the path that he started us down," said Muratore, 62, who attended an Obama rally in Columbus on Monday.
Romney backer Anastasia Loupakos, voting in Iowa City on Monday, said Romney was "the one to turn our economy around."
"I can't stand the thought of Barack as president for four more years," she said. "I couldn't stand him spending all of our money. I feel like he's destroying more jobs than he's creating."
After a long campaign that cost record sums and spawned far more political ads than ever before, Americans were showing fatigue at the end. A Pew Research Center poll released Monday showed 47 percent of Americans followed news about the election closely last week, down from 52 percent a week earlier.