John McCain on Tuesday questioned Bill Clinton's credentials to discuss the mental health of prisoners of war after the former president told an audience in Aspen that it's just a matter of time before a former POW snaps and relives the nightmare of his imprisonment.

McCain spent five and a half years in Hanoi as a prisoner of war during the Vietnam War.

"I don't know where he gets his expertise," the presumptive Republican presidential candidate told FOX News, adding that his fellow prisoners have been some of the strongest people he knows.

Comparing Nelson Mandela, who was a political prisoner in South Africa for 27 years, to a POW, Clinton told a crowd at the Aspen Ideas Festival last week that Mandela explained to him once that he had to learn to let go of his anger toward his captors in order to be truly free.

Clinton, who has vowed to campaign for Barack Obama, then suggested that POWs could lose their cool at any moment.

"It's just like if you know anybody who's ever been a POW for any length of time, you will see you go along for months or maybe even years, and then something will happen and it will trigger all those bad dreams and they will come back, and it may not last 30 seconds," he said.

Clinton added that Mandela isn't the only one who has to release his anger.

"Every living soul on the planet has some, often highly justified anger. Everybody," Clinton said. "This is a universal lesson that all of us have to keep struggling with in our lives."

McCain has run his campaign in part on national security credentials and touts his support for a surge earlier in the Iraq war as evidence of his understanding of security challenge. Besides serving in the Navy, he has been chairman and ranking member of the Senate Armed Services committee.

But McCain's war service record has been downplayed by some Obama backers, including Gen. Wesley Clark, who recently said that getting shot down in a fighter plane doesn't make one fit to serve as president. Clark later stepped back from that remark after Obama said no one's military service should be devalued.

Former Clinton White House counsel Lanny Davis said Bill and Hillary Clinton have too high a regard for McCain to say something underhanded about him.

"Let me think about it. No," Davis, a FOX News contributor, said, answering a question whether it could have been an intentional slight. "Under no circumstances ... was President Clinton saying anything in any way disparaging about the great service" of McCain.

McCain on Tuesday brushed off the latest remarks by Clinton.

"I don't know how to respond to that except to say that some of the greatest moments of my life was I had the great honor of serving in the company of heroes and observing a thousand acts of courage and compassion and love, and those that I know best and love most are those that I had the honor of being led by and served with, who inspired me to do things I never would have been capable of," said McCain, adding that Americans are more interested in the economy, jobs, housing and education.

"Whatever they want to do is fine," he added of Obama's backers.

Click here to see Bill Clinton talking about Nelson Mandela at the Aspen Ideas Festival.