Former U.S. president Bill Clinton was on the campaign trail Saturday night with U.S. President Barack Obama as the race for the White House enters its final days before Tuesday's election.

Mr. Clinton, his voice hoarse from his exhaustive campaigning for Mr. Obama, said he has literally given “my voice in the service of my president.” More than 24,000 people came to the outdoor arena in Bristow, Virginia, to hear the current and former presidents.

Mr. Obama and his Republican challenger Mitt Romney are crisscrossing the country as the campaign winds down.

Polls indicate the two candidates are virtually tied in the key states of Colorado, Florida, Ohio and Virginia. The president holds a slight edge in Ohio and Virginia, while Mr. Romney is ahead in Colorado, and the two candidates are tied in Florida.

The four states are the key so-called “swing states” critical to gaining the 270 electoral votes needed to win the White House.

At a morning rally in the swing state of New Hampshire, Mr. Romney repeated his criticism of President Obama's record, telling supporters that the president has not been able to deliver on the promises he made.

On Sunday, Mr. Romney holds another rally in Iowa and travels to Ohio, Pennsylvania and Virginia. His agenda for Monday includes trips back to Virginia, Ohio and New Hampshire, as well as a morning stop in Florida. He will spend election night in Boston, Massachusetts.

President Obama campaigns again in Ohio Sunday, as well as New Hampshire, Florida and Colorado. Monday, he returns to Wisconsin and Iowa and holds yet another event in Ohio before heading to his hometown of Chicago, where he will be on election night.