MONTPELIER, Vt. — The chairman of the Judiciary Committee said today he won’t support an attorney general nominee, Judge Michael Mukasey, potentially derailing his confirmation over complaints that he hasn’t taken a full enough stand against torture.

“No American should need a classified briefing to determine whether waterboarding is torture,” Senator Leahy, Democrat of Vermont, said. He planned an afternoon news conference to make the announcement in Burlington.

Sliding support among the panel’s Democrats makes it less likely the full Senate will vote to send Judge Mukasey to a Justice Department that has been leaderless for weeks. Four other Democrats on Mr. Leahy’s panel, which will decide Tuesday whether to approve Judge Mukasey’s confirmation, have already said they will not support him.

Once viewed as a sure thing, Judge Mukasey’s nomination was threatened during hearings last month in which he repeatedly refused to say whether he considers the simulated drowning interrogation technique known as waterboarding to be a form of torture.

Torture is considered a war crime by the international community and waterboarding has been banned by the American military, but CIA interrogators are believed to have used the technique on terror detainees as recently as a few years ago.

Judge Mukasey has called waterboarding personally “repugnant,” but said he did not know enough about how it has been used to define it as torture. He also said he thought it would be irresponsible to discuss it since doing so could make interrogators and other government officials vulnerable to lawsuits.

“I am eager to restore strong leadership and independence to the Department of Justice,” Mr. Leahy said. “I like Michael Mukasey. I wish that I could support his nomination. But I cannot. America needs to be certain and confident of the bedrock principle— deeply embedded in our laws and our values — that no one, not even the president, is above the law.”

Judge Mukasey, a retired federal judge, was nominated in September to replace Alberto Gonzales, who resigned after months of questions about his honesty in congressional testimony and whether he allowed the Justice Department to become too entwined in White House politics.

Judge Mukasey needs support from at least one Democrat on the Senate Judiciary Committee for his nomination to be sent to the full Senate for a vote. The four Democrats who sit on the panel and already have said they will oppose him are: Senator Biden of Delaware, Senator Kennedy of Massachusetts, Senator Durbin of Illinois and Senator Whitehouse of Rhode Island.