WASHINGTON, March 27 — The Senate went on record for the first time on Tuesday in favor of a withdrawal date from Iraq, with Democrats marshaling the votes they needed to deliver a forceful rebuke to President Bush’s war policy.

By a vote of 50 to 48, with a few crucial votes shifting in favor of the Democratic position, the Senate rejected a Republican effort to strip from the military spending bill any mention of a withdrawal date. The legislation will now move forward with a nonbinding goal of beginning a gradual withdrawal of American troops from Iraq within 120 days of the measure’s enactment, with a pullout by March 31, 2008.

“When it comes to the war in Iraq, the American people have spoken, the House and Senate have spoken,” said Senator Richard J. Durbin of Illinois, the No. 2 Democrat in the Senate. “Now, we hope the president is listening.”

Senators still must vote on the overall legislation this week, and then their bill must be reconciled with a House measure passed Friday. The House voted 218 to 212 for a binding measure requiring the president to bring most American combat troops home from Iraq by September 2008.

A few minutes after the vote on Tuesday in the Senate, the White House repeated its vow to veto any legislation containing a withdrawal date. The Senate action increases the likelihood that Congress and Mr. Bush will engage in a confrontation over the financing of the war.

The outcome of the Senate vote took both parties by surprise. Republicans were stung by the defection of Senator Chuck Hagel of Nebraska, who has not supported a timetable for withdrawal before although he is his party’s most outspoken critic of the war in Congress.

“There will not be a military solution to Iraq,” Mr. Hagel declared. “Iraq belongs to the 25 million Iraqis who live there. It doesn’t belong to the United States. Iraq is not a prize to be won or lost.”

The Democrats also gained the vote of Senator Ben Nelson, a Nebraska Democrat, who voted against a withdrawal date just two weeks ago.

“People want our troops home,” Mr. Nelson said.

The two other senators who crossed party lines were Gordon H. Smith, an Oregon Republican, who supported the withdrawal date, and Mark Pryor, an Arkansas Democrat, who opposed the plan. Senator Joseph I. Lieberman, independent of Connecticut, voted with the Republicans.

The Senate vote was seen as a victory for Democrats, if only because Republicans had already agreed not to stand in the way of the legislation by mounting a filibuster that requires 60 votes to pass a bill. Republican leaders said they preferred to allow Mr. Bush to veto the bill, rather than use procedural maneuvers to block the measure, which would provide $122 billion for military operations in Iraq and Afghanistan.

The impassioned debate on Tuesday illustrated that majorities in each party remained hardened in their views. Vice President Dick Cheney, who also serves as president of the Senate, arrived in the Capitol a few minutes before 5 p.m. to be on hand in the event of a tie. Two senators were not present: Mike Enzi, Republican of Wyoming, and Tim Johnson, Democrat of South Dakota, who is in rehabilitation after a brain hemorrhage.

As the debate opened Tuesday, several Republicans spoke forcefully against the notion of setting a specific date — even a nonbinding one — to get troops out of Iraq. Several Republican senators argued that the Congressional debate over Iraq has failed to factor in new developments in the country, where military commanders report signs of progress.

“This bill should be named the Date Certain for Surrender Act,” said Senator John McCain, an Arizona Republican. “A second-year cadet at West Point could tell you that if you announce when the end will be, it’s a recipe for defeat.”

Mr. McCain, who had planned to be away from the Capitol on Tuesday campaigning for his presidential bid, turned to a large map of Iraq positioned next to him on an easel as he made his argument. He pointed to areas around the country where Gen. David H. Petraeus, the American commander, has reported some success in his new military plan.

“We cannot give up,” Mr. McCain said, “just as we are starting to turn things around in Iraq.”

The Senate action on Tuesday alters the dynamics of eventual negotiations between the House and Senate to reconcile their different approaches.

Had Republicans prevailed, the Senate bill would have had no timetable, while the House version required a withdrawal no later than Sept. 1, 2008. Democratic leaders were considering whether they should eliminate the timeline entirely before sending the measure to Mr. Bush, but with both chambers approving a time frame, it is now almost certain a final measure will include some requirements for withdrawal.

Dana Perino, the deputy White House press secretary, said the president was “disappointed that the Senate continues down a path with a bill that he will veto and has no chance of becoming law.” She said Congress should allow General Petraeus’s mission to succeed “by providing our troops the funding they need — not by mandating failure.”

But Senator Edward M. Kennedy, Democrat of Massachusetts and a war critic since the conflict began four years ago, said the combination of the House and Senate votes was momentous. He said it showed how far the Democratic Congress had come toward removing troops since the beginning of the year, adding that political and policy momentum was on their side.

“Rather than continuing to defy the will of the American people and Congress by threatening to veto this legislation,” Mr. Kennedy said, “President Bush should put the Iraqis on notice.”

Three more Republicans who have expressed serious reservations about the course of the war — Senators Susan Collins of Maine, Norm Coleman of Minnesota and John W. Warner of Virginia — sided with their colleagues in trying to strip the timetable from the spending legislation. All three senators are facing tough re-election fights next year.

Ms. Collins said she was more troubled by the requirement that the administration begin removing troops within 120 days of the legislation rather than the March 2008 deadline for having most of the military out.

“I don’t think it is wise to have an abrupt withdrawal from Iraq,” said Ms. Collins, who said she was willing to wait until August to see if the continuing troop increase improves conditions there. “This doesn’t mean I support an unending commitment of our troops in Iraq. I don’t.”

Mr. Coleman said he believed that the Democratic plan was intended to score political points and that establishing such a public timeline for getting out of Iraq was militarily ill-advised.

Mr. Warner, who has criticized the administration’s conduct of the war, said he remained committed to changing policy in Iraq, but not by imposing Congressional timetables on American troops.

“It would be the bugle of retreat,” Mr. Warner said. “It would be echoed and repeated from every minaret through Iraq: the coalition forces have decided to take the first step backward. We cannot send that message. Not at this time.”