WASHINGTON (AP) – A rebellious Senate committee defied President Bush on Thursday and approved terror-detainee legislation he has vowed to block, deepening Republican conflict over a key issue in the middle of congressional campaigns.

Senator Warner, chairman of the Armed Services Committee, pushed the measure through his panel by a 15-9 vote, with Mr. Warner and two other Republican lawmakers joining Democrats. The tally set the stage for a showdown on the Senate floor as early as next week.

Earlier in the day, Mr. Bush had journeyed to the Capitol to try nailing down support for his own version of the legislation.

“I will resist any bill that does not enable this program to go forward with legal clarity,” Mr. Bush said at the White House after his meeting with lawmakers.

The president’s measure would go further than the Senate package in allowing classified evidence to be withheld from defendants in terror trials, using coerced testimony and protecting American interrogators against legal prosecution for using methods that violate the Geneva Conventions.

The internal GOP struggle intensified along other fronts, too, as Colin Powell, secretary of state during Bush’s first administration, declared his opposition to the president’s plan.

“The world is beginning to doubt the moral basis of our fight against terrorism,” Mr. Powell, a retired general who is also a former chairman of the Joint Chiefs of Staff, wrote in a letter.

Mr. Powell said that Mr. Bush’s bill, by redefining the kind of treatment the Geneva Conventions allow, “would add to those doubts. Furthermore, it would put our own troops at risk.”

Firing back, White House spokesman Tony Snow said Mr. Powell was confused about the White House plan. Later, Mr. Snow said he probably shouldn’t have used that word.

“I know that Colin Powell wants to beat the terrorists too,” he said.

Countering Mr. Powell’s letter, the administration produced one from the current secretary of state to Mr. Warner. In it, Condoleezza Rice wrote that narrowing the standards for detainee treatment as Mr. Bush has proposed “would add meaningful definition and clarification to vague terms in the treaties.”

In his committee’s vote, Mr. Warner was supported by Republican Senators McCain, Graham and Collins. Messrs. Warner, McCain and Graham had been the most active senators opposing Mr. Bush’s plan, and the vote by the moderate Ms. Collins underscored that there might be broad enough GOP support to successfully take on Mr. Bush on the floor of the Republican-run Senate.

As the battle mushrooms, it threatens to undermine campaign season assertions by the administration that it has shown a steady hand on security matters and that it should be trusted over Democrats on the issue.