The 56-43 vote fell four shy of the 60 votes needed to cut off debate on the bill, co-sponsored by Sens. Patrick Leahy, D-Vt., and Arlen Specter, R-Pa.

The vote was a blow for human-rights groups that say a current ban on habeas corpus petitions could lead to the indefinite detention of individuals wrongfully suspected of terrorism.

President Bush and conservative Republicans counter that the ban, enacted last year, was necessary to stem the tide of legal protests flooding civilian courts.

Most Republican senators backed the administration. Besides Specter, the other Republicans who voted with the Democrats were Chuck Hagel of Nebraska, Richard Lugar of Indiana, Gordon Smith of Oregon, Olympia Snowe of Maine and John Sununu of New Hampshire.

The change in law would have applied to the roughly 340 men held at Guantanamo. Many of them have been held for more than five years without being charged. The Bush administration has said that indefinite detention of enemy combatants who threaten the U.S. is necessary in an age of terrorism.

Congress enacted a law last year that establishes tribunals, made up of three military officials, to review such petitions. Sen. Lindsey Graham, R-S.C., a military lawyer who helped write the law, said the military is best able to determine who's an unlawful enemy combatant.

Graham said that under the Leahy-Specter bill, detainees could pick judges from courts around the country and demand the presence of witnesses from the battlefield.

"That's never been done in any other war, and it should not be done in this war," Graham said.

Leahy responded that people being held indefinitely without charges should be able to assert in court that they were mistakenly picked up. If a detainee is being lawfully held, the government can easily overcome the claim by presenting "the preponderance of the evidence," he said.