The Senate again took up the Iraq debate Wednesday, beginning with an amendment to a defense authorization bill that would require troops in Iraq to spend the same amount of time resting off the battlefield as they spend in Iraq.

The proposal doesn't expressly change war policy, but is seen as a de facto change because the Pentagon supposedly doesn't have enough forces to deploy more troops while newly-returned forces enjoy down time at home.

The measure is seen as the Democrats' strongest effort to tie President Bush's hands on war policy as they attempt to push forward on campaign promises to end the Iraq war and bring troops home. Similar language gained 56 votes in the Senate in July, and Democrats believe they may now have the four extra votes necessary to break a Republican filibuster.

Democrats so far have been unable to make gains in changing war policy through legislation. Despite mounting opposition earlier this year, the president sent an additional 30,000 troops this year as part of a plan to help stabilize Iraq.

Citing security gains, Bush announced last week that he intends to bring 5,700 troops home by the end of this year with a goal of bringing troop numbers in Iraq back to pre-surge levels of approximately 130,000 by mid-summer next year.

If the "dwell time" amendment from Sen. Jim Webb, D-Va., beats a filibuster, it is expected to be vetoed by the president and does not have enough support to override the veto.

"The purpose of this amendment, again, is to put a safety net under our active duty military and our (National) Guard and reserve while these debates are taking place," Webb said Wednesday on the Senate floor.

But some key Republican senators are opposed to the amendment.

Sen. John McCain, R-Ariz. — who also is running for president — said the bill violates constitutional separation of power provisions.

"It clearly violates the principles of separation of powers. Congress has no business in war time passing a law telling the Department of Defense which of its fully trained troops it can and cannot use in carrying out combat operations," McCain said from the Senate chamber.

Sen. John Warner, R-Va., former chairman of the Senate Armed Services Committee, announced on the Senate floor that he has decided to vote against the amendment by his state's junior senator. Warner supported Webb's amendment when the Senate debated and voted on the measure in mid-July, but said Wednesday after a meeting with the Joint Chiefs of Staff and receiving an awaited letter from Defense Secretary Robert Gates that he has been effectively convinced that "they cannot put into force (Webb's) amendment without causing severe problems" should another national emergency arise.

The troops debate has re-emerged after an abrupt change of heart by the Senate leadership on Iraq plans. On Tuesday, Majority Leader Harry Reid told reporters that he no longer intended to offer incentives regarding Iraq legislation to gain Republican votes, and instead would move ahead with more partisan bills.

Reid, D-Nev., said he would bring up a plan by Michigan Sen. Carl Levin that would place a binding deadline on the president to end U.S. combat presence by next summer.

The Senate also is expected to consider an amendment by Iraq war opponent Sen. Russ Feingold, D-Wis., and co-sponsored by Reid, that would cut funding for combat missions in Iraq on June 30.

Neither proposal is expected to make it out of the Senate. The underlying bill the amendments are being linked to is the must-pass 2008 National Defense Authorization Act.

The Associated Press contributed to this report.