WASHINGTON – President Bush invoked executive privilege Monday 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 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.

Retorted House Judiciary Committee Chairman John Conyers:

“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 Michigan Democrat said in a statement.

The exchange Monday 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.”

Fielding was responding to a 10 a.m. EDT deadline set by the Democratic chairmen, Sen. Patrick Leahy of Vermont and Conyers, 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, Fielding refused to comply. He said he was acting at 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,” Fielding wrote. “In such circumstances, we will not be undertaking such a project, even as a further accommodation.”

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.

The privilege claim on testimony by former aides won’t necessarily prevent them from appearing under oath this week, as scheduled.

Leahy said that he expected Taylor, Bush’s former political director, to testify as scheduled before the Senate panel on Wednesday. Fielding tried to head off any possibility that she would tell the story the president believes is protected under the privilege.

“I respectfully request that you inform Ms. Taylor that the president has directed her not to provide this testimony,” Fielding wrote in a letter Monday to Neil Eggleston, her lawyer. Eggleston did not immediately respond to a request for comment.

But Taylor was included on e-mails about the firings released earlier this year by the Justice Department, and Leahy believes she could answer questions about those and other matters.

The House Judiciary Committee scheduled Miers’ testimony for Thursday, but it was unclear whether she would appear, according to congressional aides speaking on condition of anonymity because negotiations were under way.

The probe into the U.S. attorney firings was only one of several Democratic-led investigations of the White House and its use of executive power spanning the war in Iraq, Bush’s secretive wiretapping program and his commutation last week of I. Lewis “Scooter” Libby’s prison sentence.

Fielding’s letter welcomed lawmakers back to town with a clear indication that relations between Congress and the White House had soured during the break.

Bush’s counsel cloaked his tough rejoinder to the Democratic committee chairmen in gentlemanly language, but his message was unequivocal: the White House won’t back down, and believes the congressional legal argument to be far weaker than its own and its attitude less appealing.

Fielding dismissed the chairmen’s attempt to “direct” the White House to provide the legal underpinning of Bush’s executive privilege claims and a detailed listing of the documents he is withholding. He said the White House already has provided its legal argument and so does not need to do so again – and won’t.

“We are aware of no authority by which a congressional committee may ‘direct’ the Executive to undertake the task of creating and providing an extensive description of every document covered by an assertion of Executive Privilege,” he wrote. Fielding suggested that asserting executive privilege on the testimony comes as a result of this impasse and the lack of good faith it demonstrates on the part of Congress.

More broadly, Fielding suggested that the congressional inquiry into the entire matter of the U.S. attorneys’ dismissals has no constitutional basis, in large part because the president has sole authority to hire and fire federal prosecutors.

“Although we each speak on behalf of different branches of government, and perhaps for that reason cannot help having different perspectives on the matter, it is hoped you will agree, upon further reflection, that it is incorrect to say that the President’s assertion of executive privilege was performed without ‘good faith,'” Fielding’s letter said.

Associated Press Writer Jennifer Loven contributed to this story.