U.S. Secret Service agents hold open the door as President Obama steps out of his vehicle to board Air Force One at Andrews Air Force Base in suburban Washington on Monday, Dec. 10, 2012, as he traveled to Michigan to visit the Daimler Detroit Diesel plant in Redford. (AP Photo/Charles Dharapak)

Photo by: Charles Dharapak

U.S. Secret Service agents hold open the door as President Obama steps out of his vehicle to board Air Force One at Andrews Air Force Base in suburban Washington on Monday, Dec. 10, 2012, as he traveled to Michigan to visit the Daimler Detroit Diesel plant in Redford. (AP Photo/Charles Dharapak)