WASHINGTON — Breaking with his party on a key social issue, Fred Thompson says he will not run on a plank of the 2004 Republican platform calling for a constitutional amendment banning abortion. The former Tennessee senator cited his federalist views in opposing the amendment, saying he wants to see Roe v. Wade overturned and the question of abortion returned to the states.

“I think people ought to be free at state and local levels to make decisions that even Fred Thompson disagrees with,” Mr. Thompson said yesterday on NBC’s “Meet the Press.” “That’s what freedom is all about.”

Asked directly if he would run on the 2004 party platform, which supports a constitutional amendment, he replied, “No.” Mr. Thompson said such a move would go even further than a reversal of Roe v. Wade and that “I don’t think would be the way to go.”

The GOP presidential hopeful said his federalist position extends to gay marriage, where he has called for a constitutional amendment targeting so-called judicial activism but said he would not stand in the way if individual state legislatures voted to legalize the practice. The amendment he supports would prohibit judges from altering the definition of marriage without approval from state lawmakers, but it does not go as far as the Federal Marriage Amendment backed by many evangelical Christians, which would explicitly define marriage as between a man and a woman.

Those positions could hamper Mr. Thompson’s efforts to cast himself as a more consistent social conservative than his two leading rivals for the Republican nomination, Mayor Giuliani and Mitt Romney. They could also help the surging former Arkansas governor, Mike Huckabee, who has moved into second place in some polls in Iowa, the crucial first voting state.

Mr. Huckabee, a former Baptist minister, responded to Mr. Thompson’s abortion comments yesterday by reiterating his support for what is known as the “human life amendment” and pledging to push for it as president. “My position on the sanctity of life has always been clear and consistent,” he said in a statement to The New York Sun. “No candidate has a stronger record on the sanctity of life than I do.”

The dean of Regent University’s Robertson School of Government, Charles Dunn, said yesterday that Mr. Thompson had “shot himself in the foot,” particularly when it comes to wooing the Christian right.

“Fred Thompson is having difficulty getting traction, and he didn’t help himself with that response,” Mr. Dunn said.

Mr. Thompson may benefit, however, from a practical political reality: Most anti-abortion advocates acknowledge that a constitutional amendment has little chance of passing Congress even with presidential support. “The votes just aren’t there,” the executive director of the National Right to Life Committee, David O’Steen, said. He thus had no problem with Mr. Thompson’s position, which focuses on overturning Roe v. Wade through the courts.

“He’s a pro-life candidate, and he’s got a strong pro-life voting record,” Mr. O’Steen said.

Mr. Thompson’s appearance on “Meet the Press” was his first on the venerable interview program since he became a presidential candidate. On foreign policy, he largely supported staying the current course in Iraq and opposed any timetable or deadline for withdrawing American troops. Like most of his Republican competitors, he said a military strike on Iran may be necessary, but he struck a less hawkish tone than Messrs. Giuliani or Romney have in recent weeks, cautioning that an attack “may backfire.”

“There may be a backlash,” he said. “It’s risky.”

Mr. Thompson echoed the leading Democratic presidential contenders by stopping short of a pledge that Iran would not develop a nuclear bomb while he was president. He promised only to “do everything that I could to prevent it from happening.”

Also yesterday, Mr. Thompson said he was unaware of the past drug-dealing convictions of one of his top advisers, Philip Martin, which were disclosed in the Washington Post on Sunday. A co-chairman of Mr. Thompson’s campaign, Martin pleaded guilty to selling 11 pounds of marijuana in 1979 and four years later pleaded no contest to cocaine trafficking and other charges. Mr. Thompson praised Martin as a friend but said he hadn’t decided whether to keep him on his campaign.