President Barack Obama and first lady Michelle Obama arrive at a campaign event, Wednesday, Aug. 15, 2012, in Davenport, Iowa, during a three day campaign bus tour through Iowa. (AP Photo/Carolyn Kaster) U.S. Senate candidate Barack Obama, a state senator from Illinois, give the keynote address at the Democratic National Convention on Tuesday, July 27, 2004, in Boston. (AP Photo/Ron Edmonds) President Barack Obama walks from Marine One on the South Lawn of the White House in Washington, Wednesday, Aug. 29, 2012, after returning from campaigning. (AP Photo/Susan Walsh) President Barack Obama gestures during a rally in Charlottesville, Va., Wednesday, Aug. 29, 2012. (AP Photo/Steve Helber) President Obama gestures to supporters as he takes the stage during campaign stop on the campus of Colorado State University in Fort Collins, Colo., on Tuesday, Aug. 28, 2012. (AP Photo/David Zalubowski) President Barack Obama waves as he takes the stage during campaign stop on the campus of Colorado State University in Fort Collins, Colo., on Tuesday, Aug. 28, 2012. (AP Photo/David Zalubowski) (President Barack Obama speaks during an interview with The Associated Press) at the White House, Thursday, Aug. 23, 2012, in Washington. Obama talked about the presidential race and Republican challenger Mitt Romney in the exclusive AP interview before heading off to a long weekend with his family at Camp David, the secluded presidential retreat in the Maryland mountains. His comments come ahead of the GOP convention opening Aug. 27, 2012, in Tampa, Fla. (AP Photo/Carolyn Kaster) President Barack Obama speaks to supporters during a campaign stop, Wednesday, Aug. 22, 2012, in North Las Vegas, Nev.. (AP Photo/Isaac Brekken) President Barack Obama speaks during a campaign event at Truckee Meadows Community College, Tuesday, Aug. 21, 2012, in Reno, Nev. (AP Photo/Carolyn Kaster) President Barack Obama shakes hands as he arrives for a campaign event at Truckee Meadows Community College, Tuesday, Aug. 21, 2012, in Reno, Nev. (AP Photo/Carolyn Kaster) President Barack Obama gives the thumbs-up to the crowd during a campaign event at Truckee Meadows Community College, Tuesday, Aug. 21, 2012, in Reno, Nev. (AP Photo/Carolyn Kaster) President Barack Obama reaches out to shake hands after speaking at a campaign event at Capital University, Tuesday, Aug. 21, 2012, in Columbus, Ohio (AP Photo/Carolyn Kaster) Democratic presidential nominee, Sen. Barack Obama, D-Ill., his wife, Michelle, and daughters Malia, and Sasha, are joined on stage by his running mate, Sen. Joe Biden, D-Del., and his wife, Jill, after Obama's acceptance speech at the Democratic National Convention in Denver, Thursday, Aug. 28, 2008. (AP Photo/Chris Carlson) President Barack Obama speaks during a campaign event at Morningside College, Saturday, Sept. 1, 2012, in Sioux City, Iowa. (AP Photo/Pablo Martinez Monsivais) President Barack Obama, center, greets patron watching college football during an unannounced visit to Bob Roe's Point After Pizza during an unscheduled visit, Saturday, Sept. 1, 2012, in Sioux City, Iowa. (AP Photo/Pablo Martinez Monsivais) Keynote speaker Barack Obama, candidate for the Senate from Illinois, speaks to delegates during the Democratic National Convention at the FleetCenter in Boston, Tuesday, July 27, 2004. (AP Photo/Charlie Neibergall) Vice President Joe Biden arrives to deliver the commencement address during a graduation and commissioning ceremony at the U.S. Military Academy in West Point, N.Y., Saturday, May 26, 2012. (AP Photo/Mike Groll) Democratic presidential nominee, Sen. Barack Obama, D-Ill., gives his acceptance speech at the Democratic National Convention in Denver, Thursday, Aug. 28, 2008. (AP Photo/Ron Edmonds)