WASHINGTON – Senate Democrats scored a surprise victory Tuesday in their bid to force President Bush to end the Iraq war, turning back a Republican amendment that would have struck a troop withdrawal plan from emergency military funding legislation.

Attempts by Republicans, including Maine’s Susan Collins and Olympia Snowe, to scuttle the nonbinding timeline failed 50-48, largely along party lines.

The defection of a prominent Republican war critic, Sen. Chuck Hagel of Nebraska, sealed the Democrats’ win. Hagel, who opposed identical withdrawal language two weeks ago, walked onto the Senate floor an hour before the late afternoon vote and announced he would “not support sustaining a flawed and failing policy. It’s now time for the Congress to step forward and establish responsible boundaries and conditions for our continued military involvement in Iraq.”

Democratic leaders believe the 50-48 victory greatly strengthens their negotiating position as they prepare to face down a White House that Tuesday reiterated its threat to veto the legislation. The Senate vote also represented the first time since Democrats took control of Congress in January that a majority of lawmakers have supported binding legislation to bring home U.S. troops.

The Senate withdrawal provision, which sets a March 31, 2008, target for ending combat operations, is tucked into a $122 billion package to fund operations in Iraq and Afghanistan, a must-pass bill that Democrats view as their best shot at forcing Bush to change direction. The withdrawal language was nearly identical to a Senate resolution that failed by a 50-48 vote two weeks ago.

Top House and Senate Democrats remained uncertain about the outcome of the vote when they convened for a joint leadership meeting Tuesday morning and were convinced that defeat of the Senate’s proposed timeline would force negotiators to soften the House language, which sets a firm deadline of Aug. 31, 2008, for the removal of combat forces. But they concluded a Democratic victory would give them no reason to compromise, according to House Democratic leadership aides.

Speaking to reporters, House Majority Leader Steny Hoyer, D-Md., was conciliatory, but only to a point.

“We ought to reach out to the president and say, ‘Mr. President, this is not a unilateral government. It is a separation of powers, and the Congress of the United States … has taken some action. You obviously disagree with that. Where are the areas of compromise?”

Senate Majority Leader Harry Reid of Nevada said he was skeptical of proceeding too quickly. “Of course we should reach out to the White House, and I’m happy to do that,” he said. But he added, “They have been very uncooperative to this point. Hopefully, they will cooperate with us…. I would like to have a bill that he wouldn’t veto.”

Senate GOP leaders remained confident that Bush ultimately would prevail. “I expect the president to get the money for the troops, to get this bill in large measure like he wants it,” predicted Senate Minority Leader Mitch McConnell of Kentucky. “It may take two tries to get there, but I think that’s very likely going to be the final outcome.”

Democrats are just as convinced that they have the momentum on the issue. “This is not one battle. It’s a long-term campaign,” said Sen. Chuck Schumer of New York. “Every time we have a vote like this, it ratchets up the pressure on the president and on many of those of his party.”

Under the Senate bill, which is slated for a final vote on final passage as early as today, certain U.S. forces would remain in Iraq after the March 31, 2008, target date to conduct counterterrorism, training and security operations. But troop withdrawals would begin within four months of enactment.

The White House has strongly protested both the House and Senate bills, issuing a series of veto threats. “This bill assumes and forces the failure of the new strategy even before American commanders in the field are able to fully implement their plans,” the administration said in a statement Tuesday.

Democrats and Republicans largely hung together in the Senate vote, with only Hagel and Sen. Gordon Smith, R-Ore., voting to preserve the withdrawal provision, and Sen. Mark Pryor of Arkansas the only Democrat to break ranks. Yet on both sides, several senators remained undecided until the roll was called, and Vice President Dick Cheney was on hand to break a tie in the case of a deadlock.

Sen. Olympia Snowe, R-Maine, said she was torn between her desire to send a strong message to the president that a change of course is needed, and her uneasiness about wading into war policymaking. “Clearly it’s frustrating,” she said of the grim conditions in Iraq. “On the other hand, you don’t want to telegraph to the enemy a moment in time” for leaving. Snowe wound up voting with her party.

Sen. Susan Collins said Congress should wait until fall to see whether the troop surge yields results, “then Congress should consider all options including a redefinition of our mission and a gradual but significant withdrawal of our troops next year.”

Because troop funding is at stake, Republicans have decided to forgo maneuvers that could draw out the Senate debate or block final passage, tactics the GOP had used successfully in previous Iraq showdowns. Some GOP senators even floated the idea of introducing Sen. Hillary Clinton’s Iraq legislation as an amendment to the spending bill, in a bid to make political mischief. The Clinton proposal would cap troop levels, start a phased withdrawal and cut off Iraqi security funding under some circumstances, and so far has attracted no co-sponsors.