WASHINGTON — President Bush invoked executive privilege yesterday to deny requests by Congress for testimony from two former aides about the firings of federal prosecutors.

The White House, however, did offer again to make former counsel Harriet Miers and one-time political director Sara Taylor available for private, off-the-record interviews.

In a letter to the heads of the House and Senate Judiciary panels, White House counsel Fred Fielding insisted that Mr. Bush was acting in good faith and refused lawmakers’ demand that the president explain the basis for invoking the privilege.

“You may be assured that the president’s assertion here comports with prior practices in similar contexts, and that it has been appropriately documented,” the letter said.

The House Judiciary Committee chairman, Rep. John Conyers, responded, “Contrary what the White House may believe, it is the Congress and the courts that will decide whether an invocation of executive privilege is valid, not the White House unilaterally.”

The exchange yesterday was the latest step in a slow-motion legal waltz between the White House and lawmakers toward eventual contempt-of-Congress citations. If neither side yields, the matter could land in federal court.

In his letter regarding subpoenas the Judiciary panels issued, Fielding said, “The president feels compelled to assert executive privilege with respect to the testimony sought from Sara M. Taylor and Harriet E. Miers.”

Mr. Fielding was responding to a 10 a.m. EDT deadline set by the Democratic chairmen, Senator Leahy of Vermont and Mr. Conyers of Michigan, for the White House to explain it’s privilege claim, prove that the president personally invoked it and provide logs of which documents were being withheld.

As expected, Mr. Fielding refused to comply. He said he was acting at Mr. Bush’s direction, and he complained that the committees had decided to enforce the subpoenas whether or not the White House complied.

“The committees have already prejudged the question, regardless of the production of any privilege log,” Mr. Fielding wrote. “In such circumstances, we will not be undertaking such a project, even as a further accommodation.”

Mr. Leahy also questioned the explanation.

“I have to wonder if the White House’s refusal to provide a detailed basis for this executive privilege claim has more to do with its inability to craft an effective one,” he said in a statement.