U.S. troops interrogating terrorism suspects don't know which techniques are permitted and Congress owes it to them to establish clear standards, Senate Republicans said Wednesday, opening a politically volatile debate over the treatment of detainees.

The White House (search) opposes legislation that would impose restrictions on the Pentagon's detention, interrogation and prosecution of prisoners, arguing that it would tie the president's hands in wartime.

Despite a veto threat, Sens. John McCain (search), R-Ariz., and Lindsey Graham (search), R-S.C., are trying to tack that legislation onto the $440 billion military spending bill. Votes could come as early as Wednesday night.

McCain's amendment would ban the use of "cruel, inhuman or degrading treatment or punishment" against anyone in U.S. custody and require all U.S. troops to follow procedures in the Army Field Manual when they detain and interrogate suspects. Graham's amendment would define "enemy combatant" and put into law procedures for prosecuting detainees at Guantanamo Bay.

"Confusion about the rules results in abuses in the field. We need a clear consistent standard," McCain, a prisoner of war during the Vietnam War, said on the Senate floor.

Graham, an Air Force judge for 20 years, added: "We have let the troops down when it comes to trying to give them guidance in very stressful situations."

Opposing the effort, Sen. Ted Stevens (search), R-Alaska, said that requiring all U.S. troops to follow procedures in the Army manual is not practical in the current war environment. "The techniques vary upon the circumstances and the physical location of people involved," he said.

Backed by Senate Armed Services Committee Chairman John Warner, R-Va., McCain and Graham offered the same proposals in the summer as the Senate worked on a bill setting Pentagon policy. But Senate Majority Leader Bill Frist, R-Tenn., scuttled that bill in part because of White House opposition to the detainee proposals.

In July, the White House sent Vice President Dick Cheney (search) to Capitol Hill to personally lobby McCain, Graham and Warner to drop the effort. This time, the White House approached Senate Republicans sympathetic to their position and asked them to work against the amendments.

As they did before, Democrats also plan to continue to push their own proposal that would establish an independent commission to investigate allegations of prisoner abuse. The Pentagon already has done several of its own investigations and argues that another would be redundant.

But Sen. Carl Levin of Michigan, the top Democrat on the Armed Services Committee, said those reviews weren't thorough enough. "This is a rich target for a true investigation," he said Wednesday. He accused the White House of issuing a "false threat" to veto the bill over detainee amendments.

McCain, Graham and Warner decided that standards for handling detainees were needed in light of allegations of mistreatment at the Navy's Guantanamo Bay prison in Cuba and the abuse scandal at Abu Ghraib prison in Iraq.

The high-profile Senate debate comes as fresh allegations of prisoner abuse surface and support builds for Republican-sponsored amendments.

Since July, a list of retired generals and admirals backing the effort has doubled from 14 to 28.

"It is now apparent that the abuse of prisoners in Abu Ghraib, Guantanamo and elsewhere took place in part because our men and women in uniform were given ambiguous instructions," the retired officers said in a letter dated last month. "Our service members were denied clear guidance, and left to take the blame when things went wrong. They deserve better than that."

Last month, Human Rights Watch, a U.S. rights organization, reported that soldiers in the Army's elite 82nd Airborne Division systematically tortured Iraqi detainees in 2003 and 2004. The Pentagon says it's investigating.

Army Capt. Ian Fishback (search) of the 82nd Airborne was one of the soldiers who claimed that he had heard about widespread prisoner abuse while serving in Iraq. He was on Capitol Hill this week to meet with senators, including McCain and Levin.

Last week, a federal judge in New York ordered the release of dozens more pictures of prisoners being abused at Abu Ghraib, rejecting government arguments that the images would provoke terrorists and incite violence against U.S. troops in Iraq.