President Barack Obama has become the first sitting US president to vote early, casting his vote in his home town of Chicago 11 days before the presidential election on 6 November.
The move was part of his campaign's wider effort to encourage early voting, with many states holding open in-person polls this week.
First Lady Michelle Obama voted by absentee ballot on 15 October.
It is estimated that 7.2 million Americans have already cast early ballots, and that about 35% of the electorate will have already voted by polling day.
Mr Obama is on a two-day campaign marathon across eight states. His challenger, Mitt Romney, is in Ohio, a swing state that could hold the key to the White House.
Because the US election is a state-by-state contest, a presidential candidate must win key battlegrounds like Ohio, Virginia and Florida, which do not reliably vote for either party. No Republican has ever won the White House without taking Ohio.
The BBC reports that a new national poll says Mr Romney has 50%-47% support among likely voters.
The survey, produced for ABC News by Langer Research Associates, finds that when asked about which candidate they would trust more to handle the economy, 52% backed Mr Romney versus 43% for Mr Obama - the first time either candidate has held a clear lead on the issue.