In an open rebellion against President Bush, a Republican-controlled Senate committee voted through its own bill reaffirming the Geneva Conventions and establishing clear protection for the rights of detained terrorist suspects.
The move, by the powerful Armed Forces Committee, came just hours after Mr Bush went in person to Capitol Hill to urge Republicans to support the far tougher legislation the administration is proposing, which the President says is vital for an effective waging of the "war on terror".
It splits the party down the middle on an issue which could play a crucial part in the upcoming mid-term elections, in which Republicans are struggling to hold their majorities in both the Senate and House of Representatives.
Among the demands of the Bush bill are a narrow definition of the Convention's all-important Article 3 on the humane treatment of prisoners and immunity for US interrogators from possible prosecution for war crimes. It would also set rules for terrorist trials that would permit hearsay testimony - yet allow prosecutors to withhold key evidence from defendants.
But some of the most senior Republicans were having none of it - including the committee's chairman, John Warner, as well as John McCain, a former prisoner of war and favourite for the party's 2008 White House nomination, and Lindsey Graham, Senator for South Carolina and a former military lawyer.
All three joined with Susan Collins of Maine, another moderate Republican, and with committee Democrats in a 15-9 vote for the Committee's own bill.
A showdown now looms when the full Senate takes up the detention issue, probably next week.
Further underlining divisions in the party, the previous secretary of state, Colin Powell - who resigned in strained circumstances at the end of Mr Bush's first term - went public with his opposition to the President's plan. "The world is beginning to doubt the moral basis of our fight against terrorism," Gen Powell, a former chairman of the Joint Chiefs of Staff, wrote in a letter. Redefining what is permitted under the Geneva Conventions "would add to those doubts. Furthermore, it would put our own troops at risk."
In a stinging rejoinder, Tony Snow, the White House spokesman, described Gen Powell as "confused" in his understanding of what the White House was trying to do. "The President will not accept something that shuts down the programme for interrogating detainees." For his part, John Negroponte, the director of national intelligence, has warned that the Senate bill would almost certainly mean the end of the CIA's involvement in interrogating and detaining terror suspects.
Last week, Mr Bush for the first time confirmed the existence of secret CIA camps, as the Pentagon formally banned some of the most notorious techniques, including "water-boarding" and sexual humilation of detainees. The camps are now empty, after the transfer last week of 14 top al-Qa'ida suspects to the prison at Guantanamo Bay, Cuba.
Mr Bush made crystal clear the CIA retained the right to hold and interrogate prisoners - despite the international outcry over the camps, and over the US's practice of "rendition", whereby it secretly hands over detainees to their home countries, where they are likely to be tortured, and worse.
Whatever the outcome, the Republican rift threatens to undermine claims by the Bush administration that it has shown a steady hand on security matters and that Americans should should trust it, not the Democrats, when they vote in November.