President Barack Obama and Republican rival Mitt Romney engaged in a frenzied cross-country blitz of the remaining battleground states, with both sides predicting victory in a race that remains too close to call.
National opinion polls showed a race for the popular vote in Tuesday's election so close that only a statistically insignificant point or two separated the two rivals.
The final national NBC/Wall Street Journal Poll, conducted from November 1-3, showed Mr Obama getting the support of 48% of likely voters, while Mr Romney receives 47%. The poll had a margin of error of 2.55 percentage points.
A majority of polls in the battleground states - especially in the Midwestern states of Iowa, Wisconsin and Ohio - showed Mr Obama with a slight advantage, giving him an easier path to the 270 electoral votes needed for victory. No Republican has won the White House without carrying Ohio.
Under the US system, the winner is not determined by the nationwide popular vote but in state-by-state contests, making "battleground" states that are neither consistently Republican nor Democratic extremely important in such a tight race. Mr Romney and Mr Obama are actually competing to win at least 270 electoral votes. The electoral votes are apportioned to states based on a mix of population and representation in Congress.
Mr Obama's campaign was mobilising a massive get-out-the-vote effort aimed at carrying the Democrat to victory. He had a full schedule on Sunday, with campaign stops in New Hampshire, Florida, Ohio and Colorado.
The president caught a few hours of sleep back at the White House on Saturday night before hitting the campaign trail again on Sunday morning. After Marine One lifts off from the South Lawn, Mr Obama will not return to the executive mansion again until after election day.
Mr Romney's campaign was projecting momentum and banking on late-breaking voters to propel him to victory. The Republican was cutting away briefly on Sunday from the nine or so competitive states that have dominated the candidates' travel itineraries to make a late play for votes in Pennsylvania, a Democratic-leaning state. Mr Romney will also campaign in Iowa, Ohio and Virginia.
The final frenzy of campaigning comes in the wake of Superstorm Sandy that devastated the US East Coast. It gave Mr Obama a chance to jump into action as commander in chief and left Mr Romney struggling to strike the right tone.
More than 27 million Americans have already voted in 34 states and Washington DC. Mr Obama holds an apparent lead over Mr Romney in several key states such as Iowa and Nevada. But his advantage isn't as big as the one he had over John McCain four years ago, giving Mr Romney hope that he could make up that gap in Tuesday's voting.