(Updates with Mukasey letter, response)

By Thomas Ferraro

WASHINGTON, Oct 30 (Reuters) - Michael Mukasey, U.S. President George W. Bush's nominee to be attorney general, said on Tuesday he does not know if any waterboarding interrogation methods that may be used by the United States constitute unlawful torture. But he vowed to find out.

Failing to placate critics seeking a definite denunciation of such simulated drowning, Mukasey said if confirmed by the Senate, "I will review any coercive interrogation techniques currently used."

"If, after such a review, I determine that any technique is unlawful, I will not hesitate to so advise the president and will rescind or correct any legal opinion of the Department of Justice that supports use of the technique," the retired judge and former prosecutor wrote the Senate Judiciary Committee's 10 Democrats.

Critics have accused the United States of torturing suspects in the war on terrorism, with the CIA reportedly using waterboarding after the Sept. 11 attacks.

Despite Bush's assurances that he prohibits torture, it's unclear how detainees are treated since he has refused to disclose interrogation techniques.

Committee Chairman Patrick Leahy, a Vermont Democrat, said he was unsatisfied with Mukasey's letter, written in response to questions submitted to him after his confirmation hearing earlier this month.

"I remain very concerned that Judge Mukasey finds himself unable to state unequivocally that waterboarding is illegal and below the standards and values of the United States," Leahy said.

Leahy said he will await Mukasey's written responses to other questions before scheduling a committee vote on whether to send the nominee to the Senate for confirmation as the chief U.S. law enforcement officer.

CONFIRMATION LIKELY

Despite the firestorm over torture, Mukasey is still likely to be confirmed by the 100-member, Democratic-led Senate, party aides said.

But while it was initially believed he would get near unanimous approval, up to half of the 51-member Senate Democratic caucus may now oppose him, aides said.

Assistant Senate Democratic Leader Dick Durbin said: "Judge Mukasey makes the point that in the law, precision matters. So do honesty and openness. And on those counts, he falls far short."

Democrats had initially hailed Mukasey.

They saw him as an independent-minded former judge who could help restore confidence in the Justice Department after the stormy tenure of former Attorney General Alberto Gonzales, who resigned under pressure last month and had been denounced as a White House tool.

But Mukasey faced a barrage of criticism at his confirmation hearing, particularly regarding his view of waterboarding and what some saw as his excessively broad view of presidential powers.

Mukasey testified that torture violates U.S. law, but that he did not know if waterboarding was torture. He explained he had not been briefed on U.S. interrogation methods.

Earlier on Tuesday, the three leading Democratic presidential contenders announced their opposition to Mukasey.

Sens. Hillary Clinton of New York and Barack Obama of Illinois, along with former Sen. John Edwards of North Carolina, said they were troubled by Mukasey's refusal to denounce waterboarding as torture.

After Mukasey sent his letter to the committee, another Democratic presidential contender, Sen. Joseph Biden of Delaware, announced his opposition to the nominee.

Senate Republicans accused Democrats of "obstructionism," noting Mukasey, nominated by Bush on Sept. 17, is still awaiting a Senate vote more than a month later.

"This is the longest time an attorney general nominee has been delayed in 20 years," said Senate Republican leader Mitch McConnell.

(Additional reporting by Richard Cowan)

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