Mukasey argued that giving Congress a copy of the FBI 302 report on Cheney would "significantly impair" the Justice Department's ability to investigate wrongdoing by future White House officials. Presidents and vice presidents would be reluctant to submit voluntarily to FBI interviews because there would be "an unacceptable risk" that their accounts would eventually become public, he contended in a letter to Bush recommending that the president invoke the privilege. (Both Bush and Cheney had agreed to be interviewed by Fitzgerald and FBI agents working for him. Waxman had initially subpoenaed Bush's FBI 302, as well, but the chairman dropped the request in an effort to encourage White House cooperation).

Justice Department spokesman Brian Roehrkasse disputed the contention that the arguments used by Mukasey to protect Cheney's FBI report were novel. He cited a 1986 legal opinion in which the Reagan Justice Department refused to turn over closed files from an independent counsel probe on the grounds that their disclosure might impair "prosecutorial decision-making in future cases."

The idea of applying this to "potential" future criminal investigations, as opposed to future decisions to prosecute in cases already underway, "is simply a particular variation on the same general concept," Roehrkasse said.

But a number of former federal prosecutors and legal scholars said that Mukasey's argument that future White House officials wouldn't cooperate with the Justice Department if Cheney's 302 report were to be publicly disclosed seemed a stretch. (The legal claims were prepared in part by Office of Legal Counsel chief Stephen Bradbury, whose legal opinions on interrogation and torture have come under fire from Congress).

"Creative is a good word to describe it," said Mark Rozell, another executive-privilege expert who is a professor at George Mason University's School of Public Policy, about the attorney general's contention. "This is really an argument to protect the White House's own political interests and save it from embarrassment."

As a practical matter, White House officials—including presidents and vice presidents—must cooperate with Justice Department criminal investigations involving their administrations, noted Michael Bromwich, a former federal prosecutor who investigated White House wrongdoing during the Iran-contra affair and later served as the Justice Department's inspector general. The alternative to submitting voluntarily to FBI interviews is simple: officials would invariably receive grand-jury subpoenas—and pay a rather high political, if not legal cost—if they refused to cooperate. "In the real world, high-level White House officials don't have the choice of not submitting to FBI interviews," Bromwich said.

Investigators for Representative Waxman, the chairman of the House oversight panel whose staff prepared the subpoenas, noted other problems with Mukasey's argument. Former attorney general Janet Reno agreed to turn over to Congress closed Justice Department files from the campaign-finance investigations into the Clinton White House in the 1990s, including FBI interviews with both President Clinton and Vice President Al Gore. Yet, as one staffer noted, that didn't stop Bush or Cheney from submitting to FBI interviews by Fitzgerald's team. (Indeed, Bush himself publicly ordered everybody in the White House to cooperate when the Plame probe began.)

The White House move left Waxman and his team momentarily stymied. The California congressman began his probe of the Plame affair shortly after the Democrats took back control of the House in January 2007, claiming that there were still a host of key questions about the matter (such as why no White House officials were fired or reprimanded for disclosing classified information) that were left unanswered by Fitzgerald's more narrowly focused criminal probe. (Fitzgerald ultimately prosecuted Libby for perjury and obstruction—not any underlying crime for leaking Plame's status as a CIA agent.)