WASHINGTON — Ignoring threats and warnings from President Bush, a defiant Senate committee Thursday approved legislation that would ban abusive CIA interrogations and make it easier for terrorist suspects to defend themselves at trial.

The split in Republican ranks widened as former Secretary of State Colin Powell joined the dissidents against his former boss.

The GOP-led Armed Services Committee voted 15-9 to send their legislation to the full Senate. Four Republicans — including Sen. Lindsey Graham of South Carolina — backed the bill, as did the panel’s 11 Democrats.

Graham was joined by GOP Sens. John Warner of Virginia, the committee’s chairman; John McCain of Arizona; and Susan Collins of Maine.

“The world is beginning to doubt the moral basis of our fight against terrorism,” Powell wrote to McCain.

Bush’s opposition includes some heavy-hitters: Powell is former chairman of the Joint Chiefs of Staff; McCain was a prisoner of war in Vietnam; Warner is a former secretary of the Navy; Graham is a judge in the Air Force reserves.

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

Thursday, Bush made a rare visit to the Capitol to lobby House Republicans for his approach and declared “the American people will be in danger” if lawmakers continue to defy him.

The Senate bill would ban abusive techniques the Bush administration doesn’t consider torture, such as “water-boarding,” which simulates drowning.

Although the military has banned a long list of abusive interrogation tactics, the prohibitions don’t apply to the CIA. Bush said CIA interrogators should have wide latitude, as long as they don’t engage in torture.

“In order to protect this country, we must be able to interrogate people who have information about terrorist attacks,” Bush told reporters after his closed-door visit with House Republicans. “I will resist any bill that does not enable this program to go forward with legal clarity.”

Powell endorsed the Senate Republicans’ efforts to ensure suspected terrorists receive basic protections provided by the Geneva Conventions, an international agreement governing the treatment of prisoners of war. The basic protections, spelled out in a provision known as Common Article 3, prohibit “outrages upon personal dignity,” including “humiliating and degrading treatment.”

Powell agreed with other retired senior military officers who wrote Warner contending that Bush’s approach undermines support for the war on terrorism and encourages abusive treatment of captured Americans.

The legislation under debate would revamp the ground rules for dealing with terrorist suspects, from interrogation to detention to trial before military tribunals. Some of the most controversial issues involve the treatment of detainees in secret CIA prisons.

The legislation also would change the rules of evidence for suspected terrorists facing trial before military tribunals.

One of Graham’s chief arguments is that classified information can be shared with the accused without jeopardizing national security.

The Senate committee bill would make it easier for defendants to see the evidence against them and would tighten restrictions on use of hearsay evidence.

Terrorist suspects would have fewer legal rights than Americans in civilian courts, but would have more than what Bush wants to give them.

The issue took on new urgency after the U.S. Supreme Court ruled terrorist suspects are entitled to basic protections under the Geneva Conventions.

Senate Majority Leader Bill Frist, R-Tenn., said he would decide next week whether to permit a vote on the dissidents’ bill or substitute a White House-backed alternative. Lawmakers are under pressure to finish the legislation before their planned Oct. 6 recess for the fall election season.

Bush brushed off Powell’s opposition, but White House spokesman Tony Snow didn’t conceal the annoyance his defection caused within the president’s inner circle. Snow said Powell was “confused” about Bush’s goals. Later, Snow said he probably shouldn’t have used that word.

The White House sought to counter Powell’s opposition by producing a supportive letter from Secretary of State Condoleezza Rice, and released a letter of support from five top military lawyers.

“There’s more to it,” said Graham, who contacted the lawyers after reading their letter. “The concerns I’ve been expressing for weeks, they share. ... That needs to come out, too. You can’t use the letter in a vacuum.”

The Associated Press and James Rosen, who covers Washington for McClatchy newspapers in South Carolina, contributed.