Yet courts have almost always deferred to the secrecy claims; Mr. Weaver said he believed that the last unsuccessful assertion of the privilege was in 1993. Steven Aftergood, an expert on government secrecy at the Federation of American Scientists, said, "It's a sign of how potent the national security mantra has become."

Under Mr. Bush, the secrets privilege has been used to block a lawsuit by a translator at the Federal Bureau of Investigation, Sibel Edmonds, who was fired after accusing colleagues of security breaches; to stop a discrimination lawsuit filed by Jeffrey Sterling, a Farsi-speaking, African-American officer at the Central Intelligence Agency; and to derail a patent claim involving a coupler for fiber-optic cable, evidently to guard technical details of government eavesdropping.

Such cases can make for oddities. Mark S. Zaid, who has represented Ms. Edmonds, Mr. Sterling and other clients in privilege cases, said he had seen his legal briefs classified by the government and had been barred from contacting a client because his phone line was not secure.

"In most state secrets cases, the plaintiffs' lawyers don't know what the alleged secrets are," Mr. Zaid said.

More recently the privilege has been wielded against lawsuits challenging broader policies, including the three lawsuits attacking the National Security Agency's eavesdropping program  one against AT&T by the Electronic Frontier Foundation in San Francisco and two against the federal government by the American Civil Liberties Union in Michigan and the Center for Constitutional Rights in New York.

In a filing in the New York case, John D. Negroponte, the director of national intelligence, wrote that allowing the case to proceed would "cause exceptionally grave damage to the national security of the United States" because it "would enable adversaries of the United States to avoid detection." Mr. Negroponte said he was providing more detail in classified filings.

Those cases are still pending. Two lawsuits challenging the government's practice of rendition, in which terror suspects are seized and delivered to detention centers overseas, were dismissed after the government raised the secrets privilege.