As the Senate resumes debate over the Iraq War, a new Republican amendment that would require President Bush to devise an exit strategy within the next three months is competing for bipartisan support.

On Friday, Republicans Richard G. Lugar of Indiana and John W. Warner of Virginia, two of the Senate’s most influential voices on foreign and military affairs, introduced their amendment to the annual defense policy bill (HR 1585). Bush’s war policy has been — and is expected to remain — the primary focus of the debate that started last week. Senators have readied numerous amendments that deal with Iraq.

This week, Republicans are expected to again employ a parliamentary tactic that requires 60 votes for any floor action, in order to prevent a vote on a Democratic amendment written by Senate Armed Services Chairman Carl Levin of Michigan and Jack Reed of Rhode Island. It would require a withdrawal of most U.S. troops from Iraq by the end of April 2008.

Despite the likelihood of a Republican filibuster, Majority Leader Harry Reid, D-Nev., and other Democratic leaders have portrayed the expected procedural vote on the Reed-Levin amendment as the key vote of the Iraq War debate.

“There will be one clear, unequivocal vote,” Majority Whip Richard J. Durbin, D-Ill., said July 12. “This will be a moment for those Republican senators who question the policy of this administration to show that they really want change.”

Reid has dismissed as “toothless” a rival measure by fellow Democrat Ken Salazar of Colorado that would make the recommendations of the Iraq Study Group the official policy of the United States. Those recommendations call for the redeployment of U.S. combat forces to training and force protection missions.

Salazar’s measure would not set a firm date for withdrawal. Both Democratic and Republican senators have said the amendment could win enough support to be adopted.

With the Lugar-Warner amendment introduced after many senators already had left for the weekend, it was unclear how much support it may have.

The amendment is a measured piece of legislation that attempts to chart a third way between Democratic demands for a speedy withdrawal and Republican support for Bush’s strategy of increasing troop strength to allow the Iraqi government to take political steps that would stabilize the country. Bush must report to Congress on the success of his strategy by Sept. 15.

The amendment essentially requires Bush to come up with an alternative plan if his “surge” strategy fails by September.

It would require Bush by Oct. 16 to provide Congress with a plan for the redeployment of U.S. forces in Iraq and a change in their current combat mission to guarding Iraq’s borders, training its security forces, fighting al Qaeda in Iraq, and protecting U.S. installations. The measure recommends that Bush design plans that can be implemented by Dec. 31.

The amendment also requires the intelligence community to produce by Sept. 4 a new national intelligence estimate on prospects for Iraq’s stability. It also would require “review and revision” of the intelligence findings that underpinned the 2002 use of force resolution (PL 107-243), which authorized the Iraq War. The language says that when Bush reports to Congress in September on the progress of his war strategy, Congress “expects” him to provide lawmakers with a proposal for a new authorization of force.

“I have no illusions that what the Senate does during the amendment process to this defense authorization bill is now likely to affect the president’s decision about his strategy during the next two months,” Lugar said in a floor speech when he introduced the amendment July 13. “Sen. Warner and I have tried to approach the current situation by asking, ‘What should happen now, even if the president does not change course?’ ”

In a separate statement, Warner expressed his “sincere hope that this amendment provides a basis for bipartisan consensus.

Initial Senate reaction to the amendment was mixed. Robert C. Byrd, D-W.Va., an outspoken critic of the war, praised the measure, noting it echoed an amendment he offered with Hillary Rodham Clinton, D-N.Y., that would sunset the 2002 war authorization and require Congress to pass a new one.

“If the president believes so strongly in this ‘new mission’ in Iraq, let him make his case,” Byrd said.

But Jim Manley, a spokesman for Reid, said the amendment puts too much faith in Bush to voluntarily change course in Iraq.

“Unfortunately, Sen. Reid is not as confident in the president’s willingness to change course voluntarily,” Manley said. “In the fifth year of the war, we need strong legislation that compels the president to change course, change the mission, and begin the reduction of U.S. troops. That’s what Reed-Levin does. It is binding legislation, and that is the approach he prefers.”

Christopher J. Dodd, the Connecticut Democrat and presidential candidate, also criticized the Lugar-Warner amendment for giving Bush too much time to change his Iraq strategy. “December is a long time from now, and we simply cannot afford to stay the course until then.”

Although both Lugar and Warner have discussed the war with Bush and other senior White House officials, there was no indication that they had coordinated their legislation with the administration.

“We respect Sens. Warner and Lugar and will review carefully the language they have proposed, but we believe the new way forward strategy — which became fully operational less than a month ago — deserves the time to succeed,” said White House spokesman Tony Fratto.

Adam Graham-Silverman, John M. Donnelly, Colby Itkowitz and Josh Rogin contributed to this story.