Massachusetts Gov. Mitt Romney, widely seen as a leading competitor for McCain's centrist appeal, has forcefully endorsed Bush's position.
"I am foursquare behind President Bush," Romney said in an interview. "Sen. McCain's position is mistaken on this issue."
One key issue is a section of the Geneva Convention, which sets international standards for the treatment of wartime prisoners. Known as Common Article 3, the section bans "outrages upon personal dignity, in particular humiliating and degrading treatment."
Bush says this language is vague and leaves intelligence agents in doubt about whether some of the harsher interrogation tactics they have employed are legal. He has asked Congress to clarify the language.
McCain and his allies say Congress should not unilaterally set a definition or other nations may do the same -- to the detriment of U.S. personnel held captive.
The House is expected to easily approve the White House proposal next week. But in the Senate, McCain is expected to prevail with the support of most Senate Democrats and, according to a GOP leadership aide, at least 11 Republicans.
The dispute between McCain and Bush is especially charged because their competing views are shaped by experiences at the core of their political identities.
For Bush, a revised interrogation policy is an essential tool for what he considers the historic mission of his presidency: waging a global war on terrorism unlike any other struggle the U.S. has confronted.
But McCain sees the debate through the prism of his own defining experience: the torture he endured as a prisoner during the Vietnam War.
The faceoff is the latest chapter in a complicated political relationship. McCain was Bush's chief rival for the 2000 GOP presidential nomination. During Bush's first term, McCain was at odds with the White House on major issues, including campaign finance overhaul and the Medicare prescription drug benefit. Last winter, he fought the administration for a measure to ban use of torture in Iraq and elsewhere.
But he campaigned hard for Bush's reelection in 2004. And McCain has been a notable and loyal ally on Bush's policy in Iraq.
He has also taken several steps aimed at smoothing relations with conservatives who long have doubted his commitment to their agendas. Most prominently, he delivered the commencement speech this year at Liberty University, a Baptist school in Virginia founded by the Rev. Jerry Falwell, a leader of the Christian conservative movement.
