The two rivals, speaking to some of the largest crowds of this presidential campaign, stressed their differences on the economy, health care and more.
"You have the power," Obama told thousands of cheering supporters in New Hampshire, his first appearance of a day not scheduled to end until after midnight in the East.
Later, in Cleveland, boos from Romney's partisans turned to appreciative laughter when he joked about the possibility of defeat. The Republican nominee began a sentence by saying, "If the president were to be elected," and ended it with, "It's possible but not likely."
The most recent public opinion polls were tight for the nationwide popular vote. However, they suggested at least a slim advantage for the president in the state-by-state competition for electoral votes that will settle the contest, including Ohio, Wisconsin, Iowa and Nevada.
Romney flew to Pennsylvania, a state that last voted for a Republican presidential candidate in 1988, for his first campaign foray of the general election. Obama's aides insisted Pennsylvania was sure to favor the president. Public and private polls suggested the state was relatively close.
"The people of America understand we're taking back the White House because we're going to win Pennsylvania," Romney told a large crowd that had been waiting for hours on a cold night.
In Des Moines, Romney said he would meet regularly with "good men and women on both sides of the aisle" in Congress. Later, in Cleveland, he said of Obama, "Instead of bridging the divide, he's made it wider."
Obama had New Hampshire, Florida, Ohio and Colorado in his sights for the day.
In Florida, the president said he wants to work across party lines, but quickly added there were limits to the sorts of compromises he would make.
"If the price of peace in Washington is cutting deals that will kick students off of financial aid, or get rid of funding for Planned Parenthood, or let insurance companies discriminate against people with preexisting conditions, or eliminate health care for millions who are on Medicaid .... I'm not willing to pay that price," he said, reciting some of the charges he has leveled against Romney.
The two rivals and their running mates flew from state to state as the last of an estimated 1 million campaign commercials were airing in a costly attempt to influence a diminishing pool of voters.
More than 27 million ballots have been cast in 34 states and the District of Columbia, although none will be counted until Election Day on Tuesday.
The Associated Press contributed to this report.