The presidents reversal came after months of White House attempts led by Vice President Disk Cheney and National Security Advisor Steven Hadley -- to weaken the measure, which would prohibit the "cruel, inhuman, or degrading" treatment of any detainee in U.S. custody anywhere in the world.
The Administration had been negotiating with McCain to either drop the measure or to modify it so that interrogators, especially those working for the Central Intelligence Agency (CIA), would have significant exemptions.
Bush had previously threatened to veto the bill and Vice President Cheney lobbied hard to change the McCain proposal to give interrogators more flexibility to use a range of extreme tactics on terrorism suspects.
But in the deal worked out with the President, McCain was willing to add two paragraphs to give civilian interrogators legal protections that are already afforded to military interrogators. This means that civilians would be able to defend their use of interrogation tactics by arguing in court that a "person of ordinary sense and understanding would not know the practices were unlawful."
However, experts say that if CIA or civilian personnel believe they were being directed to use an interrogation technique that was illegal, they would be obligated to disobey the order.
"We've sent a message to the world that the United States is not like the terrorists," McCain said at his joint appearance with Bush.
He added, "We are a nation that upholds values and standards of behavior and treatment of all people, no matter how evil or bad they are. And I think that this will help us enormously in winning the war for the hearts and minds of people throughout the world."
But the deal did not garner unanimous support. Rep. Duncan Hunter, a California Republican who is chairman of the powerful House Armed Services Committee, threatened yesterday to block the legislation unless the White House provides him with a written assurance that the legislation would not interfere with the ability of intelligence officials to carry out their missions.
The Bush-McCain deal won applause from human rights groups.
"We've come a long way as a country since 9/11, and this development is a sign of that," said Tom Malinowski, Washington advocacy director for Human Rights Watch. "We've gone from a sense of 'anything goes' to a recognition that torture hurts America even more than it hurts the enemy."
But human rights advocates were already looking beyond McCains victory to a separate proposed amendment by Sen. Lindsey O. Graham, a South Carolina Republican and a former military judge, that would eliminate certain rights of detainees held at the U.S. detention facility at Guantanamo Bay, Cuba.
The Graham amendment would prevent detainees from using the U.S. courts to invoke the right of habeas corpus to contest their treatment, including claims that they have been tortured. It would also effectively allow the U.S. government to indefinitely detain people at Guantanamo based on evidence obtained through "coercion."
Tom Wilner, a lawyer who represents a group of Kuwaiti detainees at Guantanamo Bay, told the Washington Post that the Graham amendment would make McCains prohibition against torture essentially unenforceable, by giving U.S. troops an incentive to engage in coercive interrogations of detainees, without fear of being held liable.