The former New York mayor has tried to tamp down criticism in recent days after Newsday reported that Giuliani was a no-show for two of the group's meetings and instead attended paid public appearances.
"I thought it would work, but then after a month or two I realized the idea that I was possibly going to run for president would be inconsistent with that," Giuliani said during a campaign stop in Iowa.
Giuliani said the main reason he quit was that it "didn't seem that I would really be able to keep the thing focused on a bipartisan, nonpolitical resolution."
The group was headed by James A. Baker III, secretary of state under the first President Bush and former Democratic Rep. Lee Hamilton of Indiana. Among its members were former Reagan Attorney General Edwin Meese III, former Supreme Court Justice Sandra Day O'Connor and one-time Clinton chief of staff Leon Panetta.
The group issued an unanimous report calling for a gradual troop pullback in Iraq without setting firm timetables and more regional diplomacy. Its bipartisan work was hailed by members of both parties.
Giuliani, who often speaks of his leadership skills, said he decided the group was the wrong place for him.
"It was a mistake because I had an active political career that could interfere with the way in which the recommendations of the commission would be viewed," he said. "All of the other members of the commission have had distinguished public careers, but none of them were prospective candidates for office."
Giuliani campaigned in Iowa for the first time since announcing that he would skip the August straw poll, an early test of political strength. His appearance came after a series of setbacks and surprises prompted reporters' questions that overshadowed his speech on fiscal conservatism.
The chairman of Giuliani's campaign in South Carolina, Thomas Ravenel, was indicted on cocaine charges. Giuliani named former state GOP chairman Barry Wynn as a replacement.
"Any federal indictment is a very serious case," Giuliani said. "I don't know anything about it. There's no light I can shed on it. He stepped down as having anything to do with the campaign."
Giuliani also faced questions about his New York successor, Michael Bloomberg, who announced that he had switched his party status from Republican to unaffiliated, increasing speculation that he would run for president as an independent.
"I like Mike very much," Giuliani said. "I am disappointed that he left the Republican Party. I still respect what he has done as mayor."
Pressed about the prospect of a Bloomberg candidacy, Giuliani said: "He says he's not running, so I've got to take him at his word. If he does run, he has every right to do it."
Giuliani has campaigned in Iowa fewer times than the other GOP contenders. Though he leads in some national polls, a new survey in Iowa showed him trailing rival Mitt Romney and running about even with unannounced candidate Fred Thompson.
"We can compete here realistically," Giuliani said. "We haven't spent a lot of time here because we've been spending a lot of time putting it together. Once we do spend a lot of time here, I think you are going to see those polls change quite dramatically."