A hoarse-voiced Bill Clinton and Barack Obama were both feeling the effects of relentless campaigning
The US presidential candidates are heading into the final two days of campaigning with the outcome still too close to call.
Republican Mitt Romney will campaign in Iowa, Ohio, Pennsylvania and Virginia, while Barack Obama heads for New Hampshire, Florida, Ohio and Colorado.
Both candidates addressed large rallies on Saturday in key swing states.
The latest ABC News/Washington Post survey suggests the pair are level with 48% of support.
Both Mr Obama and Mr Romney are showing signs of exhaustion as they continue their daily, multiple-state visits to attract any undecided voters in the marginal battleground states that will determine the winner.
Former President Bill Clinton was also suffering as he joined Mr Obama in Virginia, addressing the rally in hoarse tones, saying he had "given my voice in the service of my president".
Obama told the 24,000 people in Bristow, Virginia, that the planning and organisation of his campaign now no longer mattered.
"The power is not with us anymore, the planning, everything we do, it doesn't matter.
It's all up to you, it's up to the volunteers...
you have got the power.
That's how democracy is supposed to be."
Obama's rally in Milwaukee, pop star Katy Perry, wearing a dress emblazoned with the Democratic slogan "Forward", helped warm up a 20,000 crowd.
Obama told them not to allow Mr.
Romney to return the US to a time when Wall St had "free rein to do whatever" it liked.
Campaigning in New Hampshire on Saturday, Mr Romney criticised Mr Obama for saying that voting would be their "best revenge" on the Republicans.
"Vote for revenge?
Let me tell you what I'd like to tell you: Vote for love of country.
It is time we lead America to a better place."
Later, in Colorado Springs, the Republican challenger told supporters that Tuesday's election would be "a moment to look into the future, and imagine what we can do to put the past four years behind us".
"We're that close right now," he said.
"The door to a brighter future is there."
The BBC's Bridget Kendall, in the bellwether state of Ohio, says the campaigning there has been at its fiercest.
No Republican has ever been elected president without first winning Ohio.
But, she asks, when there has been so much pressure on people to vote early and when all but a tiny fraction of likely voters have made up their mind, how much difference will all this frantic last-minute campaigning have?