Obama also sought to tamp down controversy surrounding comments over the weekend from a supporter. Retired Gen. Wesley Clark said in a television appearance that the Vietnam War-era military service of John McCain, Obama's Republican opponent, does not necessarily qualify him to be commander in chief.

McCain was a fighter pilot who was captured and held as a prisoner of war for more than five years.

Obama said that patriotism "must, if it is to mean anything, involve the willingness to sacrifice" and sought to distance himself from Clark's remarks without mentioning them.

"For those like John McCain who have endured physical torment in service to our country — no further proof of such sacrifice is necessary," Obama said. "And let me also add that no one should ever devalue that service, especially for the sake of a political campaign, and that goes for supporters on both sides."