After a rare all-night debate, Senate Republicans were expected on Wednesday to prevent a vote on a Democratic measure that would require U.S. troops to withdraw from Iraq by next spring.

The measure, an amendment to the annual defense authorization bill (S 1547), has been the centerpiece of a Democratic strategy to force President Bush to change course on the Iraq War. It would require Bush to begin the drawdown of U.S. forces within 120 days and to complete the withdrawal of most units by April 30, 2008.

Democrats staged the all-night debate to draw attention to Republican opposition to a simple up-or-down majority vote on the amendment by Carl Levin of Michigan and Jack Reed of Rhode Island. The Senate was scheduled to vote at 11 a.m. to end debate on the measure, but with the Republican minority insisting on a 60-vote threshold for cloture, the vote was expected to fail even though a majority of senators appeared to support the amendment.

Senators traded half-hour floor speeches all night to repeat their talking points on the plan from Senate Armed Services Chairman Carl Levin, D-Mich., and Jack Reed, D-R.I., that would require a drawdown and redeployment by April 30, 2008.

Amendment supporters said the war has dragged on long enough. “It is difficult to see the wisdom of this current strategy without holding the Iraqis accountable,” said Olympia J. Snowe of Maine, one of three Republican cosponsors of the plan, around 2:30 a.m. “The time has come to speak out on behalf of the American people to say this current strategy is unacceptable.”

Presidential candidate and Armed Services ranking Republican John McCain of Arizona mocked the idea of redeploying troops for counterterrorism as the amendment calls for. “How do you do that? I guess al Qaeda will be required to wear T-shirts that say ‘al Qaeda,’ ” he said shortly after Snowe spoke.

Democrats, knowing they have a simple majority for their plan, tried to focus on the procedural obstacle. “We are asking, we are praying to the other side of the aisle: Let us vote. Let us vote,” Michigan’s Debbie Stabenow said just after 3 a.m.

A series of procedural votes allowed the plan’s supporters some time to lobby, but judging by the sentiments senators expressed after each vote, more attitudes were hardened than changed. “These things call for lots of doodling,” Larry E. Craig, R-Idaho, said around midnight, showing off an elaborate, multicolored pattern he had drawn on the back of a coaster. “This one was three and a half speeches long.”

The debate kicked off in earnest just after 8:30 p.m., when Majority Leader Harry Reid, D-Nev., called the first of three votes across the evening that kept senators from a full night’s sleep. The votes were on motions to instruct the Senate sergeant at arms to bring a quorum of senators to the chamber. Under Senate rules, if a majority shows up, a quorum exists and the motion is not enforced. Only if fewer than 51 senators come to the floor could Capitol Police seek out and arrest members — a threshold the Senate did not approach during the debate.

After the first vote, Democrats huddled in the Lyndon B. Johnson Room near the Senate floor before marching down the Capitol steps and across Constitution Avenue, which police closed for the occasion, to a candlelight rally. The rally, sponsored by anti-war group Americans United for Change, drew several hundred reporters, Hill staffers and anti-war activists, some of whom thought the Democrats’ plan would not go far enough. Shouts of “Cut the funding!” and “Put impeachment on the table!” interrupted House Speaker Nancy Pelosi, D-Calif., who joined about 20 senators at the rally.

At the vigil, Reid rallied the activists while admitting this morning’s vote would not succeed. “I want everyone here tonight, every American from coast to coast, to know that we won’t stop fighting until we end this war. We know this debate won’t be enough. This debate won’t end the debate on the war in Iraq. It won’t end because, of all the encouraging we’ve heard from some Republicans these past few weeks, too few of them are willing to vote the right way.”

Charles E. Schumer, D-N.Y. said the debate and vote were particularly designed to pressure wavering Republicans. “It is night but we are shining the spotlight on every senator, we are shining the spotlight particularly on those who go back home and say they want a change in Iraq and come here and vote to continue and rubber stamp the president’s policies.”

Back in the Senate, members kept a sense of humor and equanimity about staying up all night. By 11:30 p.m., the line to get into the public gallery to watch the debate had disappeared. At 1:35 a.m., only about 30 people remained.

Frank R. Lautenberg, D-N.J., slated to speak after 6 a.m., said he had been working on his remarks for the floor. “Maybe I’ll get a chance to exercise — that’s if I don’t fall asleep on the treadmill,” he said.

Benjamin L. Cardin, D-Md., was looking for a card game. “I’ll play just about anything,” he said.

Several old bulls went through the motions without complaint.

“It’s no big deal,” said Orrin G. Hatch, R-Utah. “I used to cause these.”

John W. Warner, R-Va., who stole out of the cloakroom early in the evening holding a piece of pizza, said these activities were old hat. His advice to the excitable youngsters going through it for the first time: “Remember it.”

On the third vote, which began about 5:10 a.m., only 60 senators showed up, and the motion was agreed to, 37-23.

Events after the cloture vote scheduled for 11 a.m. remained unclear. When asked what comes next, Lautenberg responded, “the apocalypse.” Actually, Senate Democratic leaders were considering their options of continuing the debate, bringing several other Iraq policy amendments to the floor or moving on to less controversial amendments to the underlying defense bill.

On Tuesday, Senate Majority Whip Richard J. Durbin, D-Ill., emphasized that the Levin-Reed amendment was the focal point of the Democrats’ Iraq strategy, not any competing amendments. “All of them are interesting but none of them are effective in ending the war,” he said.

The other measures include a bipartisan amendment offered by Ken Salazar, D-Colo., and Lamar Alexander, R-Tenn., that would make it official government policy to implement the recommendations of the Iraq Study Group. They call for a drawdown of U.S. forces, a change in their mission to training and counterterrorism duties and a greater emphasis on diplomacy to resolve the war. Because the recommendations do not include any timeline for troop withdrawal, Reid has called the amendment “toothless.”

Salazar said Democratic leaders promised him a vote on his amendment, but only after a vote on the Levin-Reed measure.

Another key amendment was put forth last week by Republican moderates Warner and Richard G. Lugar of Indiana. Their measure would require Bush to present Congress with a plan by mid-October to redeploy troops by the end of the year, if his troop “surge” strategy fails to bear fruit.

On Tuesday, Reid indicated he also wanted to proceed to a vote on the Warner-Lugar language, but chided them for not supporting other withdrawal efforts. “I admire and appreciate Sen. Warner and Sen. Lugar very much speaking out. I wish they would vote as well as they talk,” he said.

The Warner-Lugar amendment has failed to garner bipartisan support. Democratic leaders object to the amendment’s delay of action until October, while Republicans object to requiring the administration to document any plans before a September report on Bush’s strategy by Gen. David H. Petraeus, the top commander in Iraq, and Ambassador Ryan Crocker, the senior U.S. civilian official.

John Donnelly and Greg Vadala contributed to this story.