WASHINGTON — The Senate reopened the emotional debate on the war in Iraq yesterday in an atmosphere of suspense about whether the rising tide of bipartisan discontent has grown strong enough to force a strategic change of course on President Bush.

Key Senate Republicans, including some longtime supporters of the war, have indicated in recent days that they may support legislation to shift the United States to a more modest mission in Iraq — even before U.S. commanders report in September on the effectiveness of the surge strategy that Mr. Bush ordered early this year. It’s not clear, however, that any measure mandating troop withdrawals can win enough votes to become law over the president’s veto.

“The tide is turning” against the war, Sen. Olympia J. Snowe, R-Maine, told reporters as debate began on the 2008 defense authorization, a budget and policy blueprint that the Democratic leadership will use as a vehicle for amendments to impose troop withdrawals and other changes on the White House. “I think the president needs to recognize it and heed the call,” Snowe added. “Obviously the surge has not worked.”

Snowe said it is “very possible” that she and other Republicans will support a centerpiece of this week’s Democratic push for a new war policy, an amendment that may require troop withdrawals to begin within 120 days, with the goal of sharp force reductions by next spring.

The authors of that plan, Sen. Jack Reed and Sen. Carl Levin, of Michigan, won only 31 votes for it last summer. In May, the Levin-Reed amendment garnered a majority of 51 votes — including from two Republicans.

Reed, just back from a weekend tour of Iraq, and Levin are talking with leading Republican skeptics of the surge about possible compromise language that might muster a veto-proof majority.

“People across the country of all political persuasions want to change the policy now,” said Reed.

Majority Leader Harry Reid, of Nevada, challenged the wavering Republicans to support binding legislation to force what he called “real change” in Iraq policy, rather than “a fig leaf” measure that might merely make recommendations to Mr. Bush. “Senate Republicans who are saying the right things on Iraq,” Reid said, should “put their words into action by voting with us to change the course and responsibly end this war.”

But White House press secretary Tony Snow said, “We are committed to letting the surge work.” Referring to the pending report on the progress of that strategy — expected in mid-September from the U.S. commander in Iraq, Gen. David Petraeus — Snow said, “At this point it would be irresponsible to say we’re going to leave before we know what the results are.”

The former chairmen of the Foreign Relations Committee, Sen. Richard Lugar, R-Ind., and of the Armed Services Committee, Sen. John Warner, R-Va., are among the key Republicans who have recently voiced their doubt about the effectiveness of Mr. Bush’s strategy. But both have also expressed concern about legislation that would reduce war spending or force the president to begin troop withdrawals by certain deadlines.

Earlier this year, Democrats succeeded in passing amendments to an urgent war spending bill that would have imposed such mandates. Mr. Bush vetoed that appropriation. Lacking the votes to override the veto, the Democrats then backed away from a second veto confrontation, passing a spending bill free of troop withdrawal deadlines and mandates.

At that point, the White House expected a lull in the Iraq debate that might last until Petraeus and Ryan Crocker, the U.S. ambassador to Iraq, issued their report on the surge and their strategic recommendations for the future.

Since then the United States has completed its injection of about 28,000 new troops into Iraq, for a total of about 160,000. Reed, a former Army officer who sits on the Armed Services Committee, reported that his Iraq tour showed him evidence of limited tactical success from the surge. He said, for example, that security around Baghdad has been improved.

But Reed stressed that his Iraq visit strengthened his longstanding belief that the Iraqi government is unwilling or unable to take the political actions necessary to reconcile majority Shiite Muslims with the minority Sunni — or to provide essential services to the country.

Any gains from the surge could be short-lived, Reed said, because troop rotation schedules will make it difficult for the Pentagon to sustain the current force level beyond next spring.

Therefore, Reed said, he and Levin will press ahead with their amendment, earlier versions of which have carried a four-month deadline for the commencement of withdrawing an unspecified number of troops. The two senators will unveil the newest version of their bill tomorrow.

Meanwhile, Democratic leaders will start debate today on an amendment by Sen. Jim Webb, D-Va., that would set strict rules on the amount of time that active duty and reserve troops must be allowed to spend at home between deployments. One likely effect of the measure would be to limit the administration’s ability to maintain current troop levels.

Majority Leader Reid declined to say what other Iraq-related measures he wants debated. To the left of the Levin-Reed amendment — which may contain non-binding language calling on the president to complete the drawdown by next spring — is a proposal by Sen. Russ Feingold, D-Wis., that would set a deadline for accomplishing the withdrawal of most U.S. troops from Iraq. To the right of Reed-Levin is Colorado Democratic Sen. Ken Salazar’s amendment to adopt last winter’s strategic recommendations by a bipartisan war study commission.

The Salazar measure may become a focus of this week’s debate because, being non-binding, it could be a rallying point for Republicans who have come to oppose Mr. Bush’s strategy but who oppose mandatory troop withdrawals.

Another potentially important amendment, by Sen. Hillary Rodham Clinton, D-N.Y., and Sen. Robert Byrd, D-W.Va., would rewrite the original 2002 resolution that authorized the war, fixing an expiration date of this fall.

jmulligan@belo-dc.com