Hagel said during an appearance on CBS's "Face the Nation" that U.S. Sen. John McCain, R-Ariz., the likely GOP nominee, was pushing the ethical envelope by questioning the Illinois senator's motivations for opposing the Iraq war.

"I think John is treading on some very thin ground here when he impugns motives, when we start to get into, 'You're less patriotic than me' and 'I'm more patriotic.' I admire and respect John McCain; we talk often. John's better than that," Hagel said.

Hagel also criticized a new McCain campaign advertisement that makes it appear Obama spends more time at the gym than with wounded U.S. troops stationed overseas.

The advertisement also appears to suggest Obama canceled a visit such troops when he was in Germancy once he learned it would not be broadcast.

"He certainly would have been criticized, if, on a political trip, to go and be accused of using our wounded troops as props for his campaign," Hagel said on the CBS news program. "I think it would have been totally inappropriate of him, on a trip financed by campaign funds, to go into a military hospital. Based on what I know; he did the right thing."