WASHINGTON – As the chief federal trial judge in Manhattan, Michael Mukasey approved secret warrants allowing government roundups of Muslims in the days after the Sept. 11 attacks.

Six years later, the man President Bush wants to be attorney general acknowledged that the law authorizing those warrants “has its perils” in terrorism cases and urged Congress to “fix a strained and mismatched legal system.”

Mukasey’s caution about the material witness law probably will please Democrats who control the Senate Judiciary Committee. At confirmation hearings set to begin tomorrow, they plan to press the retired federal judge about the Bush administration’s terrorist detention policy.

The committee chairman, Sen. Patrick Leahy, long has criticized the government’s use of the warrants. They allowed the FBI to detain, without charges, an estimated 70 people, all but one of whom was a Muslim, as witnesses after the terrorist attacks in 2001.

Leahy, D-Vt., is expected to question Mukasey about this and other issues the senator has described as arising “from this administration’s abuse of secrecy and expansion of executive power.”

A fellow Democrat on the committee, New York Sen. Charles E. Schumer, said he supports Mukasey but disagrees with some of his positions on terrorist detentions.

“We may have some disagreement on what that structure should be. But he will not try to unilaterally expropriate all of the lawmaking to the executive branch. The point is that it’s done with open debate, and Congress has to pass it,” Schumer said Friday.