WASHINGTON — Senate Democrats refused to flinch Tuesday as the chamber moved toward a rare, all-night session of debate on legislation to bring troops home this fall.

They called for sleeping cots to be rolled into a room off the Senate floor and told members to prepare for repeated votes throughout the evening. Senators even left open the possibility of dispatching the sergeant at arms to summon colleagues from their homes to the floor if lawmakers ignored the debate.

The threat was reminiscent of a 1988 debate on campaign finance reform in which Capitol police carried Senator Packwood, Republican of Oregon, into the Senate feet first shortly after 1 a.m.

The goal of the planned marathon debate was to test the patience of Republicans, who have threatened to filibuster the bill. So far, the GOP leadership has been successful at blocking anti-war legislation because Democrats don’t have the 60 votes to cut off what would become an endless debate on the war.

After months of threatened filibuster, Senator Durbin, Democrat of Illinois, said it was “time for a taste of the real thing.”

Republicans dismissed the maneuver as political theater and said the Senate should vote to forgo the debate and vote immediately on whether to advance the measure. Whether the vote comes Tuesday or Wednesday, the proposal is expected to gain a majority of senators, but not reach the 60-vote threshold.

“If we leave Iraq prematurely, jihadists around the world will interpret the withdrawal as their great victory against our great power,” Senstor McCain, the top Repubican on the Armed Services Committee, said.

The legislation, proposed by Democratic Senators Levin, of Michigan, and Reed, of Rhode Island, would order troops to start leaving in 120 days and complete the pullout by April 30, 2008. Under the bill, an unspecified number of troops could remain behind to fight terrorists, protect American assets and train Iraqi security forces.

Republicans mostly have been united against setting a timetable for troop withdrawals, but GOP leaders have told the White House their job of retaining support for the war will become exponentially harder after September. Many Republicans say they want to see substantial progress by then.

That message was relayed in a private gathering Monday at the White House of GOP congressional staffers and Bush aides trying to determine an effective strategy for communications about war policies. Mr. Bush made a surprise appearance at the meeting, telling the staffers he would not rethink his Iraq policies until after a critical military assessment in September, one participant said.

Mr. Bush also said he had no confidence in the ability of international institutions — a reference to the United Nations — to salvage Iraq if America were to withdraw, according to the participant, who spoke anonymously because the meeting was intended to be private.

Also on Monday, the chairman of the Joint Chiefs of Staff, Marine General Peter Pace, said the service chiefs were developing their own assessment of Iraq to present to Mr. Bush in September. Options include another troop buildup or maintaining current troop levels beyond September, Mr. Pace said.

Mr. Bush on Monday told Iraqi Prime Minister Nouri al-Maliki and other officials that continued American support depends on political progress in Baghdad, the White House spokesman, Tony Snow, said.