To this day, Sen. John McCain, R-Ariz., can't raise his arms above his shoulders because of injuries he suffered serving his country.
Shot down on a combat mission in Vietnam, McCain was badly injured and tortured relentlessly for five and a half years in a POW camp. That experience shaped the core of his character. Had he not been through it, he might not be a candidate for president.
That's why comments made Sunday on CBS's "Face the Nation" by an Obama supporter have kicked up a hornet's nest. Gen. Wesley Clark, Ret., a former Vietnam vet himself, said McCain had been "a hero" to him during the war years, but that his service is not something that, in itself, means he should be president.
"I certainly honor his service as a prisoner of war. He was a hero to me and to hundreds of thousands and millions of others in the armed forces, as a prisoner of war," Clark told "Face the Nation" host Bob Schieffer.
But then came the comments that got everyone so riled up.
"He hasn't held executive responsibility," he said. "That large squadron in the Navy that he commanded -- that wasn't a wartime squadron."
He continued, "I don't think riding in a fighter plane and getting shot down is a qualification to be president."
Today in Pennsylvania, McCain, the presumptive Republican presidential candidate, fired back.
"If that is the kind of campaign Sen. Obama and his surrogates want to engage in, I understand that," he said. "But it doesn't reduce the price of a gallon of gas."
University of Virginia political science professor Larry Sabato, who has written extensively on character attacks in presidential politics, characterized Clark's comments as "taboo."
"This is almost the equivalent of an attack on Obama's race by the McCain side," he said. "It's something you just don't do."
Today, the McCain campaign quickly unveiled a new Truth Squad to respond to what they see as unfair attacks. Ironically, McCain's defenders include George "Bud" Day, one of the Swift Boat Veterans for Truth, who famously trashed John Kerry's war record in 2004.
Obama, D-Ill., quickly distanced himself from Clark's comments today, noting in his patriotism speech in Missouri that questioning McCain's war record should be off limits.
"No one should ever devalue that service, especially for the sake of a political campaign, and that goes for supporters on both sides," Obama said, in a tacit repudiation of Clark.
"We must always express our profound gratitude for the service of our men and women in uniform. Period," Obama said.
Both candidates seem to genuinely recoil at character attacks raised by their supporters, sensing that voters are more concerned about their mortgages, high gas prices, health care and the war.
"Character issues fill a vacuum when there's nothing else to talk about. That's not a problem this year," Sabato said.
While most commentators took issue with Clark's words, some, like Karl Frisch, of Media Matters, applauded them.
"The media-driven notion that Gen. Clark somehow attacked Sen. McCain's military service is patently false. In fact, the opposite is true -- he praised it," Frisch said. "This controversy was created and fueled by a media unwilling to live up to the basic journalistic standard of accuracy and thoroughness."
Clark's comments may not be an isolated incident.
ABC News' Teddy Davis reported today that Rand Beers, national security adviser to the 2004 presidential campaign of Sen. John Kerry, D-Mass., also jabbed at McCain's war record during a Center for American Progress panel discussion in Washington, D.C.
Beers, an informal advisor to Obama's campaign, said that, since McCain was in a state of "isolation" during much of the Vietnam War, his national security experience is "sadly limited."
The controvery may well have dimmed Clark's chances of serving as Obama's running mate. Many had thought the two would make a strong ticket, with Clark shoring up Obama's vulnerability on national security issues. Suddenly, an Obama-Clark pairing seems less likely.
Although Clark was booked on "Face the Nation" independently from the Obama campaign, Clark did contact Obama's camp for talking points. Obama's aides say they gave him some talking points but that questioning McCain's military service was not among them.
ABC News' Rick Klein contributed to this report.