President Barack Obama walks from Marine One with Air Force Col. William Knight to board Air Force One, Saturday, Aug. 11, 2012, in Andrews Air Force Base, Md., en route to Chicago. (AP Photo/Carolyn Kaster)

Associated Press
President Barack Obama walks from Marine One with Air Force Col. William Knight to board Air Force One, Saturday, Aug. 11, 2012, in Andrews Air Force Base, Md., en route to Chicago. (AP Photo/Carolyn Kaster)
President Barack Obama walks from Marine One with Air Force Col. William Knight to board Air Force One, Saturday, Aug. 11, 2012, in Andrews Air Force Base, Md., en route to Chicago. (AP Photo/Carolyn Kaster)
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