Mr Clinton, his voice hoarse from a flurry of campaign events, said he had given "my voice in the service of my president".
The former president vouched for Mr Obama's economic agenda, saying he had done a good job with a bad hand.
Mr Obama, in his fourth rally of the day, sought to draw a connection between the flush economy Mr Clinton presided over and his own policies for a second term, including increasing taxes on upper income earners.
The joint rally drew more than 24,000 people to an outdoor arena in Bristow, Virginia, on a chilly November night.
Mr Obama and Mr Clinton will campaign together again on Sunday morning in New Hampshire.