WASHINGTON — The Bush administration is aggressively wielding a rarely used executive power known as the state-secrets privilege in an attempt to squash hard-hitting court challenges to its anti-terrorism campaign.

How the White House is using this privilege, not a law but a series of legal precedents built on national security, disturbs some civil libertarians and open-government advocates because of its sweeping power. Judges almost never challenge the government's assertion of the privilege, and it can be fatal to a plaintiff's case.

The government is invoking the privilege in an attempt to wipe out the heart of a lawsuit that seeks to examine rendition, the secretive and controversial practice of sending terror suspects to foreign countries where they might be tortured.

Use of the secrets privilege also could eliminate a suit by a former FBI contract linguist who charges that the bureau bungled translations of terrorism intelligence before and after the Sept. 11 attacks.

The Bush administration also is using the secrets privilege to seek dismissal of a third case not related directly to terrorism. And the administration has invoked the privilege in less sweeping ways on several other occasions.

The use of the state-secrets privilege, critics say, is part of President Bush's forceful expansion of presidential secrecy, including a more restrictive approach to releasing documents under the Freedom of Information Act; limitations on the dissemination of presidential papers and curtailment of information on individuals rounded up in the war on terrorism.

Justice Department spokesman Kevin Madden declined to discuss active cases. But he said, "The state-secrets privilege is [asserted] only after a careful determination that, were a secret disclosed, it would adversely affect national security."

The secrets privilege is an especially powerful weapon because federal judges, reluctant to challenge the executive branch on national security, almost never refuse the government's claim to confidentiality.

That is true even though a growing body of declassified documents suggests that the privilege has been used in the past to protect presidential power, not national secrets, according to Thomas Blanton, director of the National Security Archive at the George Washington University, which works to expand public access to government documents.

There's even fresh evidence that the case leading to the Supreme Court's Reynolds decision, which enshrined the secrets privilege more than 50 years ago, may have been based more on concealing negligence than preserving national security.

In claiming the state-secrets privilege, "the government always overreaches," Blanton said. "It always misleads and in some cases it lies, because it believes its authority is at stake."

That's not so, said Shannen Coffin, who oversaw state-secrets litigation at the Justice Department from January 2002 until mid-2004.

"I don't think that's even a remotely plausible claim," said Coffin, now in private practice. "It's an extremely important privilege and one the government takes extremely seriously."

The Justice Department does not tally the government's use of the privilege. According to a recent study, however, the United States successfully has asserted the secrets privilege at least 60 times since the early 1950s, and has been stymied five times.

More striking than the number of cases is the breadth of some recent demands for secrecy, say lawyers familiar with government-secrets litigation.

For example, it would erase most of Maher Arar's suit over his seizure by U.S. officials in New York in 2002.

Arar, a Canadian citizen born in Syria, said he was shackled and flown to Jordan and then Syria where he says he was abused and imprisoned for 10 months.

His case is aimed at laying bare the arrangements between governments that underpin renditions, said David Cole, a professor at Georgetown University and one of Arar's lawyers.

If the government succeeds "in invoking state secrets, they will make renditions immune from legal challenge in court," Cole said.

Attorneys argue that the demands for secrecy have gotten out of hand.

"It's not that the privilege shouldn't exist. It's become too broad and abused with very little accountability imposed by the judiciary," said Mark Zaid, a Washington attorney who is handling two of the cases in which the government is seeking dismissal of most or all of a lawsuit.

In one of those cases, Sibel Edmonds, a former FBI linguist, charged that she was fired in retaliation for questioning security lapses at the bureau.

A Washington judge in July dismissed her complaint.

Edmonds said she believes the Justice Department was concerned about potential liability in other suits.

In March 2004, a judge threw out a racial-discrimination suit brought by Jeffrey Sterling, a black ex-CIA agent, against his former employer.

Sterling, 37, said the CIA wants to head off potential liability in its treatment of other black employees.

A CIA spokesman declined to comment.

The cases of Edmonds and Sterling — plus a third involving Drug Enforcement Administration agent Richard Horn, whose suit against the CIA in 1994 for allegedly bugging his home also was quashed on state-secrets grounds — are all before appeals courts.

In a federal appeals court in Philadelphia, meanwhile, Patricia Reynolds Herring wants judges to re-evaluate a suit she filed more than five decades ago that became the modern anchor for the state-secrets privilege.

The Supreme Court ruled that the executive branch could assert a secrets privilege when "there is a reasonable danger that compulsion of the evidence will expose military matters which ... should not be divulged."

The ruling meant that the Air Force did not have to produce an accident report on a 1948 crash of a B-29 testing secret electronic equipment. Reynolds' first husband, Robert Reynolds, was one of three civilian engineers killed.

The current challenge began in 2003, after Herring and other plaintiffs' relatives obtained a copy of the accident report from a Web site selling declassified documents. They discovered that the engine fire that caused the plane to go down was linked to shoddy maintenance.

The new suit alleges the government committed fraud by citing national security in refusing to release the report.

The new suit seeks a financial settlement and does not contest the government's right to a secrets privilege. But Herring said she hopes it will lead to a healthy skepticism.

"I feel very strongly [the Reynolds] ruling was tainted," said Herring, 77, of Carmel, Ind. "My hope would be that people would be more wary and less trusting of anything that's told to them by the government. Everything is not a matter of protecting the national security."