An epic battle over executive privilege could be joined this week as the White House seeks to block two former aides to President Bush from testifying before Congress.

The Senate Judiciary Committee has subpoenaed a former legal counsel to the president, Harriet Miers, and a former White House political director, Sara Taylor, to appear on Wednesday to answer questions about their role in the firing of nine United States attorneys late last year.

Ms. Miers, an attorney and longtime adviser to Mr. Bush, is widely expected to resist the subpoena. However, the case of Ms. Taylor could present a more urgent problem for the White House. On Saturday, her attorney, W. Neil Eggleston, wrote to the Judiciary Committee indicating that she was willing to “testify without hesitation,” but that she had been advised to expect a letter from the White House “directing her not to comply with the Senate’s subpoena.”

“These contrary directions undoubtedly create a monumental clash between the executive and legislative branches of government,” Mr. Eggleston wrote. “It is unfair to Ms. Taylor that this constitutional struggle might be played out with her as the object of an unseemly tug of war.”

Mr. Eggleston’s letter, which was also directed to the White House counsel, Fred Fielding, did not say whether Ms. Taylor would appear at Wednesday’s hearing or answer senators’ questions.

The fact that Ms. Taylor, 32, left the White House in May is not likely to be pivotal in determining Mr. Bush’s privilege claim. However, by dangling the possibility that she might testify, the former aide could force the White House to go to court in the coming days rather than wait for Congress to take some action to enforce the subpoenas.

In a joint interview yesterday on CNN, the chairman of the Judiciary Committee, Senator Leahy of Vermont, and the ranking Republican, Senator Specter of Pennsylvania, said they expect Ms. Taylor to appear on Wednesday as planned.

“We’ll ask Miss Taylor when she does come before the committee this week just where she feels on this. I haven’t heard anything from Mr. Fielding or anybody else at the White House that would justify a claim of executive privilege,” Mr. Leahy said.

“We bring Ms. Taylor in and we can ask her a lot of questions that don’t invoke executive privilege at all,” Mr. Specter said.

A White House spokesman, Tony Fratto, declined to say what specific instructions were given to Ms. Taylor, but he said Mr. Bush’s attorneys believe they have ample grounds to block forced testimony by White House aides. “The president is prepared to assert privilege if the committee continues down this path of confrontation with subpoenas,” the spokesman said. “We still hold out the opportunity for the committee to reject that confrontational route.”

Mr. Fielding has offered to make White House officials available for informal, off-the-record questioning by senators behind closed doors. However, Messrs. Leahy and Specter said yesterday that the offer was unacceptable because it precluded subpoenaing the officials later if further testimony was needed.

At the White House, Ms. Taylor served largely in a subordinate role to one of Mr. Bush’s closest political advisers, Karl Rove. E-mails provided to Congress by the Justice Department show that she was involved in discussions about the dismissal of the prosecutors. Democrats contend that those firings may have been intended to punish lawyers who failed to prosecute Democrats for election fraud.

There is no definitive legal precedent addressing the scope of executive privilege in the face of a congressional investigation. However, two of the players in the current dispute played key roles in earlier high-profile fights.

While serving as White House counsel to President Reagan, Mr. Fielding dealt with a dispute over a House subpoena issued to the head of the Environmental Protection Agency, Anne Gorsuch Burford. The House voted to hold Burford in contempt, but the U.S. attorney in Washington refused to prosecute her and instead moved to quash the subpoena. A court rejected that suit, but Mr. Fielding ultimately negotiated a compromise that permitted Congress to examine, but not retain, some of the records it sought.

Mr. Eggleston fought executive privilege battles on behalf of President Clinton during an investigation of his relationship with a White House intern, Monica Lewinsky, and during an earlier inquiry into the conduct of an agriculture secretary, Alphonso Mike Espy.

Ruling on that dispute in 1997, the D.C. Circuit Court of Appeals called the power to appoint and remove executive branch officials “a quintessential and non-delegable Presidential power.”

“Confidentiality is particularly critical in the appointment and removal context; without it, accurate assessments of candidates and information on official misconduct may not be forthcoming,” the court wrote. The decision would be a great boon to Mr. Bush in the looming legal fight, except that the judges went on to say their opinion should not be applied to a conflict between Congress and the White House.

Ms. Taylor said she would abide any court rulings, but it is not clear whether the White House can prevent a former staffer from speaking out if she chooses to do so.

“My understanding is the White House cannot legally stop her from having a conversation with someone, chatting with a reporter on TV about what she knows, or going before a congressional committee as a private citizen,” a professor of public policy at George Mason University, Mark Rozell, said.

“Executive privilege lasts forever,” a former assistant chief counsel on the Senate Watergate Committee, David Dorsen, said. “It’s not up to every employee to decide for himself or herself. … If the privilege is valid, they should have a mechanism for stopping her.”