The Republican-controlled Senate voted to impose restrictions on the treatment of terrorism suspects, delivering a rare wartime rebuke to President George W. Bush.
Defying the White House, senators voted 90-9 to approve an amendment that would prohibit the use of "cruel, inhuman or degrading treatment or punishment" against anyone in US government custody, regardless of where they are held.
The amendment was added to a $440 billion military spending bill for the budget year that began Oct. 1.
The proposal, sponsored by Sen. John McCain, also requires all service members to follow procedures in the Army Field Manual when they detain and interrogate terrorism suspects.
Bush administration officials say the legislation would limit the president's authority and flexibility in war.
But lawmakers from each party have said Congress must provide US troops with clear standards for detaining, interrogating and prosecuting terrorism suspects in light of allegations of mistreatment at Guantanamo Bay and the abuse scandal at Abu Ghraib prison in Iraq.
"We demanded intelligence without ever clearly telling our troops what was permitted and what was forbidden. And when things went wrong, we blamed them and we punished them," said McCain, a prisoner of war in Vietnam.
"Our troops are not served by ambiguity. They are crying out for clarity and Congress cannot shrink from this duty," said McCain, an Arizona Republican.
The Senate was expected to vote on the overall spending bill by weeks' end. The House-approved version of it does not include the detainee provisions. It is unclear how much support the measure has in the Republican-run House of Representatives.
Rep. John Murtha of Pennsylvania, the top Democrat on the House Appropriations subcommittee on defense, is supporting McCain's legislation. Murtha could prove a powerful ally when House and Senate negotiators meet to reconcile differences in their bills.
The confrontation by members of the president's own party shows how reluctant some lawmakers are to give him unchecked wartime power as the conflict in Iraq drags on and US casualties mount. It also comes as the president seeks to show strength after weeks in which his approval rating plummeted, with Americans questioning the direction of the war, the sluggish federal response to Hurricane Katrina and the upsurge in gas prices.
Sen. Ted Stevens, an Alaska Republican, said he was concerned that McCain's legislation could inadvertently endanger the lives of people who work in classified roles. He said he hoped to fix the potential problems during negotiations with the House.
"There are some changes that have to be made if we are going to be faithful to those people who live in the classified world," Stevens said.
Also pending is an amendment by Sen. Lindsey Graham, a South Carolina Republican, that would distinguish between a "lawful enemy combatant" and an "unlawful enemy combatant." His proposal would put into law the procedures for prosecuting them at the Navy's Guantanamo Bay prison in Cuba.
Former Secretary of State Colin Powell, a retired four-star Army general, endorsed McCain's effort.
"The world will note that America is making a clear statement with respect to the expected future behavior of our soldiers. Such a reaction will help deal with the terrible public diplomacy crisis created by Abu Ghraib," Powell said in a letter that McCain read on the Senate floor.
Republican supporters say that US troops interrogating terrorism suspects do not know which techniques are allowed.
"We have let the troops down when it comes to trying to give them guidance in very stressful situations," said Graham, an Air Force judge for 20 years.
But Sen. Jeff Sessions, an Alabama Republican, said the legislation is unnecessary. "We do not have a system of systematic abuse of prisoners going on by our United States military," he said.
The White House has said Bush advisers would recommend the president veto the entire bill over the legislation. But a veto is considered highly unlikely given that Bush has never used that power.
Also, scrapping a measure that provides money for pay raises, benefits, equipment and weapons for troops while the country is fighting wars in Iraq and Afghanistan would open the president to a flood of criticism.