The vote, 49 to 42, on an amendment to a military budget bill by Senator Lindsey Graham, Republican of South Carolina, comes at a time of intense debate over the government's treatment of prisoners in American custody worldwide, and just days after the Senate passed a measure by Senator John McCain banning abusive treatment of them.

If approved in its current form by both the Senate and the House, which has not yet considered the measure but where passage is considered likely, the law would nullify a June 2004 Supreme Court opinion that detainees at Guantánamo Bay had a right to challenge their detentions in court.

Nearly 200 of roughly 500 detainees there have already filed habeas corpus motions, which are making their way up through the federal court system. As written, the amendment would void any suits pending at the time the law was passed.

The vote also came in the same week that the Supreme Court announced that it would consider the constitutionality of war crimes trials before President Bush's military commissions for certain detainees at Guantánamo Bay, a case that legal experts said might never be decided by the court if the Graham amendment became law.

Five Democrats joined 44 Republicans in backing the amendment, but the vote on Thursday may only be a temporary triumph for Mr. Graham. Senate Democrats led by Jeff Bingaman of New Mexico said they would seek another vote, as early as Monday, to gut the part of Mr. Graham's measure that bans Guantánamo prisoners from challenging their incarceration by petitioning in civilian court for a writ of habeas corpus.

So it is possible that some lawmakers could have it both ways, backing other provisions in Mr. Graham's measure that try to make the Guantánamo tribunal process more accountable to the Senate, but opposing the more exceptional element of the legislation that limits prerogatives of the judiciary. Nine senators were absent for Thursday's vote.

Mr. Graham said the measure was necessary to eliminate a blizzard of legal claims from prisoners that was tying up Department of Justice resources, and slowing the ability of federal interrogators to glean information from detainees that have been plucked off the battlefields of Afghanistan and elsewhere.

"It is not fair to our troops fighting in the war on terror to be sued in every court in the land by our enemies based on every possible complaint," Mr. Graham said. "We have done nothing today but return to the basics of the law of armed conflict where we are dealing with enemy combatants, not common criminals."

Opponents of the measure denounced the Senate vote as a grave step backward in the nation's treatment of detainees in the global war on terror. "This is not a time to back away from the principles that this country was founded on," Mr. Bingaman said during floor debate.

Senator Arlen Specter of Pennsylvania, chairman of the Judiciary Committee and one of four Republicans to vote against the measure, said the Senate was unduly rushing into a major legal shift without enough debate. "I believe the habeas corpus provision needs to be maintained," Mr. Specter said.

A three-judge panel trying to resolve the extent of Guantánamo prisoners' rights to challenge detentions sharply questioned an administration lawyer in September when he argued that detainees had no right to be heard in federal appeals courts.

The panel of the District of Columbia Circuit is trying to apply a 2004 Supreme Court ruling to two subsequent, conflicting decisions by lower courts, one appealed by the prisoners and the other by the administration.

In its June 28, 2004, decision in Rasul v. Bush, the Supreme Court ruled 6 to 3 that the Guantánamo base was not outside the jurisdiction of American law as administration lawyers had argued and that the habeas corpus statute allowing prisoners to challenge their detentions was applicable.

Under Mr. Graham's measure, Guantánamo prisoners would be able to challenge only the narrow question of whether the government followed procedures established by the defense secretary at the time the military determined their status as enemy combatants, which is subject to an annual review. The District of Columbia Circuit would retain the right to rule on that, but not on other aspects of a prisoner's case.

Detainees would not be able to challenge the underlying rationale for their detention. "If it stands, it means detainees at Guantánamo Bay would have no access to any federal court for anything other than very simple procedural complaints dealing with annual status review," said Christopher E. Anders, a legislative counsel for the American Civil Liberties Union. "Otherwise, the federal courts' door is shut."

If the measure is enacted, civil liberties groups said it would appear to render moot the Supreme Court's decision on Monday to decide the validity of the military commissions that Mr. Bush wants to try detainees charged with terrorist offenses to trial. But some legal experts said the court might be able to move ahead if determined to do so.

Under the Graham amendment, the measure would apply to any application or action pending "on or after the date of enactment of this act."

Elisa Massimino, Washington director of Human Rights First, said: "The Senate acted unwisely, and unnecessarily, in stripping courts of jurisdiction over Guantánamo detainees. Particularly now, as the string of reports of abuse over the past several years have underscored how important it is to have effective checks on the exercise of executive authority, depriving an entire branch of government of its ability to exercise meaningful oversight is a decidedly wrong course to take."

The Senate vote on Thursday came just days after senators voted, for the second time in recent weeks, to back a measure by Mr. McCain to prohibit the use of cruel and degrading treatment against detainees in American custody.

Vice President Dick Cheney has appealed to Mr. McCain and to Senate Republicans to grant the C.I.A. an exemption to allow it extra latitude, subject to presidential authorization, in interrogating high-level terrorists abroad who might know about future attacks. Mr. McCain said Thursday that negotiations with the White House on compromise language were stalemated.

In addition to Mr. Specter, Republicans voting against the bill were Senators John E. Sununu of New Hampshire, Gordon H. Smith of Oregon, and Lincoln Chafee of Rhode Island. The five Democrats voting for the bill were Senators Joseph I. Lieberman of Connecticut, Mary L. Landrieu of Louisiana, Ben Nelson of Nebraska, Kent Conrad of North Dakota and Ron Wyden of Oregon.