“Clinton clearly was more aggressive in using executive privilege than any of the modern presidents since Eisenhower,” said Mark J. Rozell, a professor at the School of Public Policy at George Mason University and the author of “Executive Privilege: Presidential Power, Secrecy, and Accountability.” “Bush has been somewhat reluctant to use it.”  But even though the legal concept has been in use for more than 200 years, its scope remains largely unsettled.  One reason is that clashes between the executive branch and Congress over the privilege, while not uncommon, seldom result in an impasse or find their way into the courts. They are, probably fittingly, worked out through political accommodation.  “What usually breaks the deadlock,” Louis Fisher, a specialist in constitutional law at the Library of Congress, wrote in 2004 in “The Politics of Executive Privilege,” “is a political decision: the determination of lawmakers to use the coercive tools available to them, and political calculations by the executive branch whether a continued standoff risks heavy and intolerable losses for the president.”  Among the tools and threats at Congress’s disposal, Mr. Fisher wrote, are negative publicity, the power of the purse, the possibility of withholding Senate confirmation for some executive officials and the possibility of holding potential witnesses in contempt.  Mr. Fielding is a veteran in battles over executive privilege. He has a reputation for nuance and flexibility, playing his cards depending on the strength of his hand.  As White House counsel in the Reagan administration in 1981, he helped head off a confrontation with a House appropriations subcommittee that wanted to hear from an aide named Martin Anderson, according to Mr. Fisher’s book. After Mr. Anderson refused to appear, the subcommittee deleted a $3 million budget request.  Mr. Fielding worked out a compromise similar to the one he offered yesterday. Mr. Anderson met “informally and off the record with the subcommittee,” Mr. Fisher wrote. Most of the requested money was restored.  Mr. Fielding successfully invoked executive privilege to block a request that he himself give sworn testimony in hearings on the nomination of Edwin Meese III to be attorney general.  This time, Mr. Fielding’s hand may not be especially strong. He faces a hostile Congress and a president weakened by an unpopular war. Even Republican allies have faulted the administration for missteps by the F.B.I. over domestic surveillance and by the Pentagon over health care for wounded soldiers. White House aides have testified before Congress relatively frequently. A 2004 report of the Congressional Research Service cites scores of examples in the last 60 years. In general, Mr. Fisher said in an interview, the likelihood that aides will testify increases with the need to respond to accusations of abuse and bad faith and with Congressional and public pressure.  The Supreme Court’s only sustained consideration of the scope of the privilege, in the 1974 decision that ordered President Richard M. Nixon to turn over the Watergate tapes to a special prosecutor, is of only limited help in understanding assertions of executive privilege in the legislative setting.  The Nixon case arose in the context of a criminal trial, and the court went out of its way to say that it “was not concerned here with the balance between the president’s generalized interest in confidentiality” and “Congressional demands for information.”  But the case did make clear that executive privilege is not absolute and must yield in the face of at least some other important interests.  The court, in a unanimous decision by Chief Justice Warren E. Burger, also said the case for executive privilege was strongest where there was a need “to protect military, diplomatic or national security secrets.”  The interest asserted in the current dispute — the need to protect confidential personnel decisions — is less strong but hardly frivolous. “It’s not a silly area in which to assert some kind of executive independence,” said Vikram Amar, a professor at Hastings College of the Law in San Francisco.  But the administration’s argument is not helped by the fact that it has already disclosed thousands of pages of documents on the subject. In the context of other protections for confidential information like the attorney-client privilege, a partial disclosure can waive the privilege.  President Bush has invoked executive privilege relatively infrequently, at least by name, legal scholars said. When he has invoked it, though, he has often expanded its scope.  He has asserted it on behalf of former presidents and closed investigations. And he allowed Vice President Dick Cheney to assert something similar in refusing to disclose information about an energy task force.  In 2004, the Supreme Court ruled for Mr. Cheney. The decision was the occasion for Justice Kennedy’s musings on the troublesome potential of the privilege. He said it should not be lightly invoked because it was a recipe for constitutional confrontation.  A version of this article appears in print on , on Page A16 of the New York edition with the headline: The White House and Congress Seem Headed Toward a Familiar Collision. Order Reprints| Today's Paper|Subscribe
Both sides are mindful that the White House possesses a powerful weapon, executive privilege, which is the constitutional equivalent of a declaration of war.