On the first day of his confirmation hearings, Mr. Mukasey said he would resign if directed by the White House to take any action he believed was illegal or violated the Constitution, winning Democratic praise. On the second day of his testimony, Mr. Mukasey sidestepped the question of whether waterboarding was torture and also suggested that the president’s Constitutional powers could supersede federal law in some cases.

Those responses stirred strong Democratic opposition, throwing his confirmation into question. Trying to stem the rising opposition, Mr. Mukasey said that while he personally found the concept of waterboarding repugnant, he could not pass judgment on whether it was illegal because he had not been briefed on administration interrogation techniques.

Senator Dianne Feinstein, Democrat of California, said she was confident that Mr. Mukasey would be nonpartisan and that his refusal to make a judgment on torture without knowing all the facts of interrogation policy should not keep him from the post.

“This man has been a judge for 18 years,” said Ms. Feinstein, who along with Mr. Schumer provided the key supporting votes to push Mr. Mukasey through the Judiciary Committee. “Maybe he likes to consider the facts before he makes a decision.”

But she was in conflict with most of her Democratic colleagues. Senator Harry Reid of Nevada, the Democratic leader, opposed the choice even though he said he was predisposed to back Mr. Mukasey.

“During his confirmation hearings, Judge Mukasey expressed views about executive power that I and many other senators found deeply disturbing,” said Mr. Reid. “And I was outraged by his evasive, hair-splitting approach to questions about the legality of waterboarding.” Republicans hailed Mr. Mukasey and accused Democrats of stalling the nomination and focusing on the torture issue to score political points. “The Department of Justice has a vital role to play in the war against Islamic terrorists, and it is critically important that it have a leader who can ensure that it fulfills its mission,” said Senator Jon Kyl, Republican of Arizona. “Judge Mukasey is this kind of leader.”

Mr. Mukasey presided over terrorism prosecutions while a federal judge in New York and has publicly backed the administration’s broad interpretation of powers granted it under the Patriot Act. Named to the bench by President Ronald Reagan in 1988, Mr. Mukasey served as a judge of the United States District Court for the Southern District of New York until 2006, the last six years of his tenure as chief judge.

Before that, Mr. Mukasey practiced law in New York City for 20 years and served as an assistant United States attorney in the federal prosecutor’s office for four years. In 1976, he joined the law firm of Patterson Belknap Webb & Tyler, where he returned three days after his retirement from the district court.