Click here to watch "Phillip Butler."

On the heels of John McCain's nomination speech at the Republican National Convention -- where he praised his country and cited his prisoner-of-war and military experience as key elements of his biography as a candidate -- came a hard-hitting ad featuring a fellow POW aiming to put all that into doubt.

Brave New PAC, a left-leaning advocacy group, and Democracy for America, which describes itself as the "nation's largest progressive political action community," are partnering to launch a 30-second spot that will run nationwide on CNN, MSNBC and ESPN beginning Sunday. The ad features POW Phillip Butler voicing his concerns about whether McCain's imprisonment necessarily qualifies him to be president. Butler went to the Naval Academy with McCain and was imprisoned with him in Vietnam. "Hell, I’m 70 years old and I’ve lived through being a prisoner of war, I’m going to tell it like it is," Butler says in the ad. "I think I can say with authority that the prisoner of war experience is not a good prerequisite for a president of the United States."

The ad doesn't stop there. With stern and stoical images of the Arizona senator on the screen, Butler questions his personality, saying "he was well-known as a volatile guy, and he would blow up and go off like a Roman candle. John McCain is not somebody that I would like to see with his finger near the red button." The ad is a snippet of a longer video in which Butler further details his concerns about McCain.

Axel Woolfolk, a spokesman for Brave New Films, said there are two main issues the group hopes viewers take from Butler's words: judgment about the war and how McCain has used his POW experience in the election; and McCain's temperament in general.

Woolfolk also said that in light of both presidential nominees’ running-mate selections, more talk has arisen about what qualifies someone to be president. "Dr. Butler wanted to speak to that as well," Woolfolk said.

This spot is just the first from Brave New PAC and Democracy for America, according to DFA Communications Director Daniel I. Medress. "We’re going to make 'maverick' a bad word," he said.