US President Barack Obama, left, is joined by former President Bill Clinton, right, and Democratic candidate for the U.S. Senate from Virginia, former Gov. Tim Kaine, center, on stage at a rally at Jiffy Lube Live arena, late Saturday night in Bristow, Va. (AP photo)

President Barack Obama greets supporters after speaking at a campaign at Jiffy Lube Live. (AP photo)

Barack Obama, center, is joined by Bill Clinton, left, and Tim Kaine, right, on stage at a rally at Jiffy Lube Live arena. (AP photo)

In the final hours of a four-state campaign day, Obama is joined by Bill Clinton, left, and Tim Kaine. (AP photo)

In the final hours of a four-state campaign day, Obama arrives at a rally at Jiffy Lube Live arena. (AP photo)

Obama speaks at a rally at Jiffy Lube Live arena, late Saturday night. (AP photo)

Obama is joined by Bill Clinton and Tim Kaine, at a rally at Jiffy Lube Live arena. (AP photo)

Barack Obama is joined by Bill Clinton, left, at a rally at Jiffy Lube Live arena. (AP photo)

Republican presidential candidate, former Massachusetts Gov. Mitt Romney, pauses during a speech at a campaign rally in Greenwood Village, in south Denver, Colo., Saturday. (AP photo)

A woman holds a U.S. flag as she and others listen to a speech by Mitt Romney, during a campaign rally in Greenwood Village. (AP photo)

Mitt Romney, speaks during a campaign rally in Greenwood Village, in south Denver. (AP photo)

Mitt Romney campaigns at Comfort Dental Amphitheater, in Englewood. (AP photo)

Mitt Romney, left, looks on as wife Ann introduces him at a campaign event at the Comfort Dental Amphitheater. (AP photo)