Democratic presidential candidate Sen. Barack Obama, D-Ill., left, arrives at Midway International Airport in Chicago, Wednesday, July 16, 2008. (AP Photo/Jae C. Hong)

** FILE ** In this June 18, 2008 file photo, Democratic presidential candidate Sen. Barack Obama, D-Ill., center, and Ret. Gen. Wesley Clark, talk with generals and other flag officers during a military and foreign affairs round table discussion in Washington. Clark, a former Democratic presidential candidate now supporting Barack Obama, said Sunday June 29, 2008 John McCain’s military service does not automatically qualify him to be commander in chief. (AP Photo/Alex Brandon)