Washington ? Faced with pressure from fellow Republicans to impose restrictions on the Pentagon’s treatment of detainees, Senate Majority Leader Bill Frist said Tuesday he would delay until September consideration of the $491 billion defense bill authorizing funding for military operations next year.

Frist’s decision reflected the challenge confronting the Bush administration as it tries to fend off efforts by several important Republican senators to give Congress a role in how the military conducts interrogations of terrorism suspects.

At the same time, GOP senators from South Dakota and Maine have objected to proposals to close military facilities in their states as part of a nationwide base restructuring and are supporting amendments that would delay any closings. With at least four Republicans – John Thune of South Dakota, Susan Collins of Maine, John McCain of Arizona and Lindsey Graham of South Carolina – leading the effort to extend debate and consider the amendments, Frist, of Tennessee, lacked the 60 votes needed to force a vote on the bill. In the end, 50 senators voted to cut off debate; 48 voted to continue it.

Frist previously had shown no urgency to bring the defense authorization bill to a vote. The Senate Armed Services Committee approved it two months ago.

The question of whether to impose restrictions on the military’s conduct of interrogations brought out the differences between the administration and some Republican senators.

Vice President Dick Cheney had been working behind the scenes to kill the amendments, setting up the confrontation with McCain, a former prisoner of war in Vietnam; Graham, who served as an Air Force lawyer for 20 years; and John Warner of Virginia, the chairman of the Senate Armed Services Committee, who supported them.

Taken together, the interrogation and base-closing amendments suggest a growing independence among Senate Republicans, as President Bush struggles with declining support for the war in Iraq as well as an investigation into the involvement of top White House aides in disclosing the identity of a CIA officer.

Supporters of the measure that would regulate the conduct of interrogations see it as a final opportunity to impose restrictions on Pentagon operations – at the Guantanamo Bay detainee center, in Cuba, and at such facilities as the Abu Ghraib prison, in Iraq – that have drawn sharp criticism and have proved embarrassing to the administration.

Roughly a dozen reports have been issued in 15 months since the scandals at the prison near Baghdad were first revealed, but none has called for dramatic, systemic change, prompting the effort by McCain and the others to step in.

One of McCain’s proposals would make the interrogation techniques outlined in the Army Field Manual the standard for handling all detainees in the Defense Department’s custody. Another would prohibit cruel, inhumane or degrading treatment of U.S.-held prisoners, whether detained in the United States or elsewhere.

Graham’s amendment would give congressional approval to the administration’s legal policies on detainees, under which some are held indefinitely, by defining in law the term “enemy combatant” and codifying the policies already in effect in the Defense Department.

The two sets of amendments – dealing with the prisoners and fighting the base-closing proposals – touched off a threat by the administration of a presidential veto.

With Congress planning to begin a summer recess this weekend, there will be no opportunity to return to the defense measure until after Labor Day – a delay that will give the administration additional weeks to pressure its Republican allies to back off their proposals.