When the Senate returns this fall and resumes debate on a defense bill, several Republican senators plan to continue trying to tack on amendments that would impose restrictions on the Pentagon's treatment of terror suspects in U.S. custody and delay proposed military base closings. There's no indication the White House will drop its veto threats against those efforts.

That could make for a confrontation fueled by members of the president's own party. Such a standoff was averted last month when Senate Majority Leader Bill Frist, R-Tenn., abruptly pulled the defense bill from the floor after efforts to kill the amendments failed.

"We look to the future," said a clearly disappointed Sen. John Warner, R-Va., chairman of the Senate Armed Services Committee that crafted the bill. "We'll take it up again in September."

Together, the looming fights over detainees and base closings suggest an increasing reluctance by some GOP lawmakers to give Defense Secretary Donald H. Rumsfeld _ and by extension Bush _ carte blanche during wartime.

As the Iraq war drags on, more Republicans could join Democrats in challenging the president, especially if the conflict's popularity among U.S. voters continues to fall as American casualties mount.

The efforts also reflect a deepening inability by Frist to keep his Republicans in line, and a burgeoning independence by some members of Bush's own party that could threaten other parts of the second-term president's agenda as he battles lame-duck status.

It's unclear how much support the detainee and base-closing amendments have.

The chief proponents of the detainee proposal _ GOP Sens. John McCain of Arizona, Lindsey Graham of South Carolina and Warner _ have credentials that boost their credibility. And lots of senators are leery of closing military bases in their home states, especially during wartime.

The defense measure, called an authorization bill, sets Pentagon spending and policy for the coming year but provides no actual dollars. Historically, Congress has used the legislation to force the Pentagon to make operational changes, creating prime conditions for a high-stakes political fight.

"The attitude by the administration and the Pentagon is we don't need an authorization bill _ that they don't do anything but tie our hands anyway," said Powell Moore, a Defense Department official during Bush's first term. "The administration has leverage by saying they'll just veto it."

That's exactly what the administration has threatened to do if the detainee or base-closing amendments are added to the bill. Following through, however, would put the president in the sticky situation of killing a measure that includes military pay raises and other benefits for troops while the country is at war.

Before it comes to that, the White House likely will try pressuring Republican sponsors of the amendments to back off.

The amendments _ by McCain, a former prisoner of war in Vietnam, and Graham, an Air Force lawyer for 20 years _ would regulate the Pentagon's interrogation and prosecution of detainees. Warner, a former Navy secretary and chairman of the Armed Services panel, supported the effort.

The trio decided Congress needed a say in the treatment of detainees amid allegations of mistreatment at Guantanamo Bay prison in Cuba and the abuse scandal at Abu Ghraib prison in Iraq. They also sought to derail growing support for a Democratic amendment creating an independent commission to investigate operations at those facilities and others.

Fearing its authority would be usurped, the Pentagon resisted any involvement by Congress. Yet intense lobbying by the White House didn't persuade McCain, Graham and Warner to drop the effort.

"Nothing has changed," McCain said.

Sen. John Thune, R-S.D., facing the loss of his state's Ellsworth Air Force Base, recruited a bipartisan group of lawmakers to support another amendment delaying the Pentagon's plan to close hundreds of military bases across the country.

Frist and the White House clearly didn't want to take a chance last month that the amendments would pass. When Frist couldn't get enough support in his own caucus to derail them, he simply put off further consideration of the defense bill until fall.

He then took up firearms legislation _ prompting Democratic accusations that the potential 2008 presidential candidate was pandering to the gun lobby.

The switch allowed Frist and the White House to avoid votes that would have put the spotlight on the president's defense policies as he faces low approval ratings in polls _ and put a damper on the GOP's celebration of legislative victories it enjoyed before Congress' recess.

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Liz Sidoti covers Congress and national security for The Associated Press.