Obama and former President Bill Clinton drew 24,000 people to an outdoor rally in Bristow, Va., on a cold Saturday night.
Clinton, his voice hoarse after a week of campaigning, said he had "given my voice in the service of my president." But that didn't stop him from launching into a 30-minute defense of Obama and his economic policies.
He also slammed Romney for his shifting positions, saying "He could be the chief contortionist for Cirque de Soleil."
Obama, who spoke second, embraced Clinton as he walked on stage. The president said at this stage of the campaign, he was largely "a prop" and the race was in the voters' hands.
"The power is not with us anymore," he said. "It's all up to you."
Obama's campaign said it had registered 1.8 million voters in key battleground states, nearly double the number of voters they registered in 2008. Campaign officials said volunteers had made 125 million personal phone calls or door knocks with voters.
Romney has also attracted large crowds in the final weekend of campaigning. His rally in Ohio on Friday drew more than 20,000 people.
The Republican nominee has been using teleprompters to deliver his final campaign speeches. He's claiming the mantle of change - and highlighting what he says was a bipartisan record as governor of Massachusetts.
In addition to Pennsylvania, Romney will campaign Sunday in Iowa, Ohio and Virginia.