The White House has decided to defy Congress's latest demand for information regarding the dismissal of nine U.S. attorneys, sources familiar with the decision said yesterday. Such an action would escalate the constitutional struggle and propel it closer to a court showdown.

Senate and House committees have directed President Bush to provide by tomorrow a detailed justification of his executive privilege claims and a full accounting of the documents he is withholding. But White House counsel Fred F. Fielding plans to tell lawmakers that he has already provided the legal basis for the claims and will not provide a log of the documents, the sources said.

The standoff suggests that neither side is prepared to budge in the fight over documents and testimony in the widening U.S. attorney investigation. Officials in both camps said no serious negotiations are taking place to resolve the dispute. Fielding plans to follow up his letter by further asserting executive privilege later this week, the sources said, directing former White House aides Harriet E. Miers and Sara M. Taylor not to testify in response to congressional subpoenas.

The two sides increasingly believe that the matter will lead congressional Democrats to seek criminal contempt citations against the White House, which could result in a protracted court battle over the contours of the president's power to shield White House deliberations. Both sides insist that the other's legal position is weak and argue that this could be one of the most important test cases in years.

The impasse is leading to "a monumental clash between the executive and legislative branches of government," Taylor's attorney, W. Neil Eggleston, wrote in a letter sent yesterday to Fielding and leaders of the Senate Judiciary Committee. "This clash may ultimately be resolved by the judicial branch."

Eggleston wrote that his client has been unfairly put in the middle of "an unseemly tug of war" in which she faces the choice of betraying a president she admires or being sanctioned by the Senate. "Ms. Taylor is willing to testify and should not face personal peril because the White House and the Senate are unable to resolve their dispute," Eggleston said in an interview.

The conflict stems from congressional investigations into the dismissal last year of nine chief federal prosecutors. The Senate and House judiciary committees issued five subpoenas seeking documents and testimony by Taylor, who until six weeks ago was the White House political director, and Miers, the former White House counsel. Bush asserted executive privilege June 28 in refusing to respond to the subpoenas for documents, and the committees then demanded that he provide a legal basis and a records log.

The log, according to the committees, should describe each document withheld, including its source, subject matter, date and recipients. White House officials viewed the request as a backdoor attempt to get sensitive information about deliberations, the sources said.

A White House spokesman declined to comment yesterday. But the White House in recent days has said that it has cooperated extensively with congressional investigators and is now standing on principle. It has also accused Congress of being more interested in investigating than legislating, noting that Democrats have held 600 oversight hearings since taking control of Capitol Hill in January.

"We've turned over 200,000 pages of documents as an administration," White House spokesman Scott Stanzel said last week. "And in that time, what they have to show for it, if you're taking a generous look at it, is six bills, six major bills passed. . . . What does Congress want to do? Do they want to pass legislation for the American people, or would they rather investigate and have politics be the course of the day?"

Senate Judiciary Committee Chairman Patrick J. Leahy (D-Vt.) said yesterday that refusing to provide a more detailed legal basis and the log would suggest that the White House is trying to cover something up or realizes it does not have a reasonable claim. He noted that the administration is willing to give some of the information previously under strict conditions.

"This latest stonewalling attempt raises troubling questions about what the White House is trying to hide by refusing to turn over evidence it was willing to provide months ago, as long as the information was shared in secret with no opportunity for Congress to pursue the matter further," Leahy said through a spokeswoman.

Lanny A. Breuer, who was a White House special counsel under President Bill Clinton during many fights with Congress and special prosecutors over executive privilege, said it is highly unusual to refuse to provide even a privilege log describing the basis for withholding documents. The Clinton White House, he said, regularly provided such logs.

"The White House position is extremely weak," Breuer said. "You can invoke privilege if it's honestly believed, but Congress has an absolute right to understand the basis on which you're claiming privilege. . . . I think the administration has decided that, at this point, they want to fight it as long as they can. It may go to court."

David B. Rivkin, a senior lawyer under Presidents Ronald Reagan and George H.W. Bush, agreed that the White House may be eager to go to court on this issue -- because Fielding believes he has a strong case, not the other way around. Congress has a weak claim to demand internal documents and testimony from the executive branch, he said, since the president has the right to fire U.S. attorneys without input from lawmakers.

"The president has pretty much absolute power in this area," Rivkin said. "I think the decision is that they might as well have a serious legal fight about it" because the administration is "highly likely to succeed."