billions on attack ads, ground games, and get out the vote efforts, and squared off in three intense debates.
Republican presidential candidate Mitt Romney speaks during a campaign event at the Newport News International Airport in Newport News, VA. AP/David Goldman
Their running mates -- Vice President Joe Biden and Republican congressman Paul Ryan -- have laid out the rationales for their bosses' aspirations.
First Lady Michelle Obama, Romney's wife Ann and countless surrogates on both sides have made the case.
Monday marks the final, last-ditch attempt by incumbent and challenger to convince the narrowing sliver of undecided voters that their policies, their platforms, their approach to leading America forward are the right ones come 2013.
And with polls showing that, for the most part, each has as equal a shot at the White House as the other, Obama and Romney will engage in unvarnished efforts to mobilize their core supporters.
A combination photo shows Mitt Romney speaking in Indianapois, Indiana, and Barack Obama at a campaign rally in Mentor, Ohio. AFP
"I need you, Ohio," Obama admitted to a 20,000-strong crowd in Cincinnati, in a state for which both candidates are fighting tooth and nail.
"And if you're willing to work with me, and knock on some doors with me, if you're willing to early vote for me, make some phone calls for me, turn out for me, we'll win Ohio. We will win this election," the President said.
Both candidates campaigned deep into the night on Sunday, with Romney too imploring his supporters to get out the vote in the handful of battleground states where the next occupant of the White House will be decided.
"We've got a little work to do in the coming days... which is to make sure we have a win on Tuesday night," the Republican nominee said at a night rally in Newport News, Virginia.
The final dash underlined the tightness of a race that is drawing to a close with the rival candidates and their aides confidently predicting victory after months of campaigning and conflicting fortunes in opinion polls.
Women hold up signs spelling "vote" on a stage being prepared for a speech by President Barack Obama at a campaign rally Aurora, in Denver. AP
As the clock ticked down to Tuesday's vote, Romney's efforts included a surprise foray into Pennsylvania, a Democratic-leaning state that Republican strategists say is breaking his way.
"We're taking back the White House because we're going to win Pennsylvania," Romney told a crowd of up to 30,000, according to US Secret Service estimates quoted by the campaign, who had gathered on a farm in frigid weather.
Obama advisers dismissed the trip as a sign of desperation from the challenger less than 48 hours from election day.
And yet a valuable character witness, former president Bill Clinton, will headline four rallies for Obama on Monday in Pennsylvania, to counter Romney's late push there.
Democrats said they were confident of Obama's small but steady lead in key swing states, but acknowledged that everything now depends on getting the vote out.
US President Barack Obama speaks during a campaign rally in Aurora, Colorado. AFP
"Ultimately, it's up to you. You have the power," Obama said at a rally in Concord, New Hampshire. "You will be shaping the decisions for this country for decades to come right now, in the next two days."
Obama flew to New Hampshire to reprise a buddy act from the night before with Clinton, which saw the popular former president place his economic legacy on the younger leader's shoulders.
On a grueling swing, the Democratic incumbent also traveled to Florida, Ohio, and Colorado before touching down in Wisconsin in the early hours Monday.
Exhaustion began to show on both candidates this past weekend as they keep up frenetic paces straight into Monday, when each performs another multi-state crisscross that would make any frequent flyer proud.
Obama begins in Wisconsin, continues to Ohio and then to Iowa, the state where Obama began his presidential run in 2008, before returning to his home town Chicago.
Romney starts the day in the biggest swing state of all, Florida. He then flies to Virginia and kingmaker Ohio one last time before returning to where his campaign began 18 months ago: New Hampshire.
Supporters hold a sign as US President Barack Obama speaks at a campaign event at the University of Cincinnati. Reuters
Campaign aides pointed at early voting advantages in Ohio and Florida as evidence that Obama is close to sealing the deal in his quest to become only the second Democrat since World War II to get a second term.
"Early vote's gone very well for us. We think we're closing with strong momentum," Obama advisor David Plouffe told ABC.
The two candidates are effectively tied in national polls of the popular vote but Obama appears to have a stronger claim to the battleground states, and if the polls are accurate, seems to be in position to win re-election.
Mitt Romney high fives supporters as he campaigns at Colorado Springs Municipal Airport in Colorado Springs, Colorado. AP