Senator Obama hit a bump in the road in his new drive to attract voters who harbor doubts about his patriotism yesterday when he was forced to denounce remarks one of his prominent supporters made minimizing the value of Senator McCain’s military service.

The flap arose over comments a former four-star general, Wesley Clark, made on a talk show Sunday arguing that Mr. McCain’s experience as a Navy pilot, including more than six years in a North Vietnamese prison, did little to prepare him for the presidency. “I don’t think riding in a fighter plane and getting shot down is a qualification to be president,” General Clark said on CBS’s “Face the Nation.”

In a conference call for reporters organized by Mr. McCain’s campaign, friends and military colleagues of the presumptive Republican nominee expressed outrage. “I was utterly shocked when I saw this yesterday, knowing Clark as I have, that he would in such a disrespectful way attack one of his fellow career military officers,” Senator Warner of Virginia said.

Several of Mr. McCain’s supporters said they needed to act quickly and decisively against attempts to challenge their candidate’s war record because it had been unfairly impugned in his 2000 presidential campaign, when he was accused of having sold out prisoners of war left alive in Vietnam.

A former Marine officer who was imprisoned at the Hanoi Hilton along with Mr. McCain, Orson Swindle, said yesterday that General Clark was “spreading rumors” about the Vietnam veteran. Mr. Swindle did not elaborate.

While Mr. McCain’s surrogates condemned General Clark’s comments, the candidate was more circumspect. “I think that that kind of thing is unnecessary,” the Arizona Republican said. “I’m proud of my record of service, and I have plenty of friends and leaders who will attest to that.”

Mr. McCain wavered when asked if he believed General Clark’s comments were part of an organized effort by the Obama campaign. “General Clark is not an isolated incident, but I have no way of knowing how much involvement Senator Obama has in that issue,” Mr. McCain said.

In a speech yesterday in Independence, Mo., Mr. Obama seemed to rebuke General Clark, though he did not single him out. “I will never question the patriotism of others in this campaign. And I will not stand idly by when I hear others question mine,” the presumptive Democratic nominee said. “We must always express our profound gratitude for the service of our men and women in uniform. Period. Full stop.”

In response to queries about General Clark’s comments, a spokesman for Mr. Obama, William Burton, issued a statement saying, “As he’s said many times before, Senator Obama honors and respects Senator McCain’s service, and of course he rejects yesterday’s statement by General Clark.”

One of those who stepped forward to defend Mr. McCain yesterday was a retired Air Force colonel and Medal of Honor winner, George Day. In 2004, Mr. Day appeared in television ads for the Swift Boat Veterans for Truth, a group that questioned Senator Kerry’s military service record in Vietnam.

Mr. Day was named to the McCain campaign’s official “truth squad” yesterday, notwithstanding Mr. McCain’s denunciation of the Swift Boat ads and the group’s salvos against Mr. Kerry, some of which were deemed inaccurate by news organizations and independent groups.

“The Swift Boat, quote, attacks were simply revelations of the truth,” Mr. Day said when asked about the issue yesterday. “What the Swift Boat campaign was about was to lay out John Kerry’s record. John Kerry’s never produced any evidence that denied that. … There is a 180-degree difference here. This one is about attempting to cast a new shadow of John McCain, a very indecent thing.”

General Clark, who until a few weeks ago was an energetic backer of Senator Clinton in the Democratic primary fight, is a somewhat unlikely figure to pursue a purported Obama campaign strategy to denigrate Mr. McCain’s war record.

However, General Clark has for months demonstrated a willingness and propensity to question the notion that Mr. McCain’s wartime service would be of much use to him as president. Answering a question from The New York Sun in March, the former NATO commander said he believed Mrs. Clinton had more useful national security experience than Mr. McCain. “Having served as a fighter pilot — and I know my experience as a company commander in Vietnam — doesn’t prepare you to be commander in chief in terms of dealing with the national strategic issues that are involved. It may give you a feeling for what the troops are going through in the process, but it doesn’t give you the experience firsthand of the national strategic issues,” he said.

In a statement sent in an e-mail to reporters last night, he reiterated remarks he made Sunday on CBS calling Mr. McCain a “hero.” However, the former general refused to back down from his contention that Arizona senator’s background is devoid of major decision making responsibility.

“That experience shows courage and commitment to our country — but it doesn’t include executive experience wrestling with national policy or go-to-war decisions,” General Clark said.

In recent weeks, other well-known Democrats have ventured into similar territory. Senator Harkin of Iowa, who was neutral in the primary but has since endorsed Mr. Obama, suggested in May that Mr. McCain’s military service and his family ties to the military gave a him a “pretty dangerous” outlook that made him less suitable as president. “I think one of, one of the problems John McCain has, is that his grandfather was an admiral, his father was an admiral. He comes from a long line of just military people, so I think his whole worldview, his life view has been shaped from a military viewpoint, and he has a hard time thinking beyond that,” he said, according to Radio Iowa.

In April, Senator Rockefeller of West Virginia, a supporter of Mr. Obama, suggested that Mr. McCain’s role as a naval aviator skewed his perspective. “McCain was a fighter pilot who dropped laser-guided missiles from 35,000 feet. He was long-gone when they hit,” he told the Charleston Gazette. “What happened when they get to the ground? He doesn’t know. You have to care about the lives of people. McCain never gets into those issues.”

Mr. Rockefeller later apologized, calling his own comments “inaccurate and wrong.”