WASHINGTON — The next big showdown here is going to be between two secretaries of state.

It covers the argument between the White House and Republican opponents of its proposal to authorize military trials for captured Al Qaeda leaders. The two sides offered dueling letters setting out the competing arguments: one from Secretary of State Powell, arguing for the dissident senators, and one from Secretary of State Rice, defending President Bush.

On a day when Mr. Bush placed a rare call on Republican House lawmakers on Capitol Hill, Senator McCain, a Republican from Arizona, released a caustic letter from Mr. Powell complaining that the White House bill would fuel doubts from the rest of the world on “the moral basis of our fight against terrorism.”

A few hours later, the White House released a similarly pugnacious letter from the former general’s successor, Ms. Rice, who wrote that the standards applied to interrogators under the president’s bill would be exactly the same as the torture ban sponsored by Mr. McCain in 2005, known as the Detainee Treatment Act.

At issue is Common Article III of the Geneva Conventions, which outlaws “cruel” and “degrading” treatment for captured prisoners of war. Mr. Bush has said the treaty’s language is too vague for America’s interrogators in the war on terrorism and that it would place them potentially at the mercy of foreign courts, where they could be tried as war criminals.

Mr. Bush said yesterday at the White House that he would “resist any bill” that does not enable the CIA’s network of secret detention centers “to go forward with legal clarity” for terrorist suspects believed to be the most dangerous.

But the president faces stiff opposition from a trio of Republican senators: a former prisoner of the North Vietnamese communists, Senator McCain; the chairman of the Senate Armed Services Committee, John Warner, a Republican of Virginia, and Senator Graham, a Republican of South Carolina. These senators have said the proposed White House legislation would legislate away America’s obligation to adhere to the treaty dictating the humane treatment of prisoners of war. As such, the White House legislation would strip American soldiers of the protections afforded them under the treaty.

While the White House yesterday played down the tensions within the Republican Party over the new legislation, they emerged yesterday on yet another issue relating to the bill.

Only a few hours after Mr. Bush visited House members, the three Republican senators joined 11 Democrats on the Senate Armed Services Committee to pass a measure that would allow suspected terrorists to see classified evidence against them if brought before military tribunals. This proposal contradicts Mr. Bush’s bill, which allows government prosecutors to withhold secret evidence from the defense counsels of suspected terrorists.

The battle over the legislation has intensified with Mr. Powell’s entry into the fray, and it appeared clear that he had some old scores to settle. The president’s former top diplomat offered the most significant presentation of the pre-war Iraq intelligence for weapons of mass destruction in Iraq before the U.N. Security Council in 2003, making the case for war. Since then, Mr. Powell has said he regretted the speech that claimed Iraq was concealing a nuclear, biological, and chemical weapons program and maintained links to Al Qaeda.

Behind the scenes, Mr. Powell has become a stern opponent of his former boss. He refused, for example, a year and a half ago to sign a letter of former Republican secretaries of state endorsing John Bolton’s nomination as ambassador to the United Nations. Mr. Powell’s chief of staff at the State Department testified in closed session to the Senate Foreign Relations Committee staff against Mr. Bolton.

Mr. Powell also kept secret that it was his former deputy, Richard Armitage, who was the initial source for Robert Novak’s column disclosing the CIA status of White House critic Joe Wilson’s wife, Valerie Plame. Mr. Armitage is one of the advisers to Mr. McCain’s informal committee planning a 2008 presidential run.

“The world is beginning to doubt the moral basis for our war against terrorism,” Mr. Powell wrote. “To redefine Common Article III would add to those doubts. Furthermore, it would put our own troops at risk.”

At the White House yesterday, President Bush shrugged off the letter. “There’s all kinds of letters coming out,”he said.”And today, by the way, active duty personnel in the Pentagon, the [Judge Advocates Generals], supported the concept that I have just outlined to you.” The military lawyers to whom the president referred had initially supported the McCain version of the anti-terror legislation.

The president’s spokesman Tony Snow said Mr. Powell had not consulted with the White House before he wrote his letter. Addressing Mr. Powell’s letter and another missive from former chairman of the Joint Chiefs of Staff, Jack Vessey, Mr. Snow said, “What’s interesting is, in a way, we agree with the two gentlemen. They don’t understand what we’re trying to do here. We’re not trying to alter or amend Common Article III. Regrettably, we hadn’t known, or we hadn’t heard from either of them. We could have had some discussions with them to lay out what we’re trying to do.”

[Yesterday, a rebellious Senate committee defied President Bush and approved the terror-detainee legislation he has vowed to block, deepening Republican conflict over terrorism and national security in the middle of the election season, the Associated Press reported.

Mr. Warner, normally a Bush supporter, pushed the measure through his Armed Services Committee by a 15-9 vote, with he and three other GOP lawmakers joining Democrats.The vote set the stage for a showdown on the Senate floor as early as next week.]