A US Senate committee has defied President George W Bush by rejecting legislation to set up trials for foreign terrorism suspects.
Some 460 suspects are believed to be being held at Guantanamo Bay
The panel voted 15-9 to back a milder bill, which Mr Bush has vowed to block.
Ex-Secretary of State Colin Powell backed Republicans opposing the measure sponsored by Mr Bush that would allow military tribunals at Guantanamo Bay.
Correspondents say the split within the party risks damaging its prospects in November's mid-term elections.
The White House wants the new Guantanamo tribunals to maintain the right to use evidence obtained through coercion and to keep elements of prosecution cases secret from those accused.
But the senators argued that Mr Bush's proposals would effectively redefine the Geneva Conventions to allow harsh treatment of detainees held at the Guantanamo Bay camp in Cuba.
They said their own version would provide fair trials and meet the demands of the US Supreme Court, that struck down Mr Bush's original plan.
Four Republican senators joined opposition Democrats on the Armed Services Committee to approve their measure.
The rebels include three prominent senators, John McCain, John Warner and Lindsey Graham, who say Mr Bush's bill would do further damage to America's moral authority.
The senators are also worried about White House efforts to reinterpret Article Three of the Geneva Conventions in order to allow tougher interrogations of suspects.
Khalid Sheikh Mohammed (above left): Alleged mastermind of 9/11; believed to be the number 3 al-Qaeda leader before he was captured in Pakistan in 2003
Abu Zubaydah: Alleged link between Osama Bin Laden and many al-Qaeda cells before his capture in Pakistan in 2002
Ramzi Binalshibh (above right): One of the alleged masterminds of 9/11
Hambali (Riduan Isamuddin): Alleged senior leader in Jemaah Islamiah (JI); wanted by Indonesia, Malaysia, Singapore and the Philippines in connection with blasts
The article in question governs detainee treatment; banning torture, violence and degrading treatment - and demands that the sick and wounded are cared for.
Senators' arguments have been joined by former secretary of state Colin Powell, who said in a letter that redefining the conventions would put US troops at risk.
In an intervention that appears to have taken the White House by surprise, Mr Powell wrote: "The world is beginning to doubt the moral basis of our fight against terrorism."
His comments were made public as President Bush made a rare personal visit to Congress in order to lobby on behalf of his bill.
Speaking afterwards, Mr Bush said the stakes were high.
"If there's not clarity, if there's ambiguity... the programme won't go forward and the American people will be endangered," he said.
Congress has two weeks to reach a compromise before it goes into recess ahead of November's mid-term elections.
Correspondents say that if it does not, the status of Guantanamo inmates will remain in limbo - and the Republican split on national security may make it harder for candidates to campaign on an issue they had hoped would be a vote-winner.
The split comes a week after Mr Bush announced plans to resume military tribunals, which were stopped in June.
If Congress approves Mr Bush's plan, the Pentagon's chief prosecutor said suspects including Khalid Sheikh Mohammed, the alleged architect of the 11 September 2001 attacks, could go on trial within months.
The US Supreme Court has ruled that Mr Bush does not have the authority to order such trials, but left the way open for the president to seek Congressional approval for their resumption.