Democrats steered the Senate into an attention-grabbing, all-night session to dramatize opposition to the Iraq war but conceded they were unlikely to gain the votes needed to advance troop withdrawal legislation blocked by Republicans.

"Our enemies aren't threatened by talk-a-thons, and our troops deserve better than publicity stunts," said Sen. Mitch McConnell of Kentucky, the Republican leader.

McConnell and many other Republicans favor waiting until September before considering any changes to the Bush administration's current policy. They have vowed to block a final vote on the Democrats' attempt to require a troop withdrawal to begin within 120 days.

"We have no alternative except to keep them in session to explain their obstruction," said Senate Majority Leader Harry Reid of Nevada.

So far, the legislation has drawn the support of three Republicans, Sens. Gordon Smith of Oregon, Olympia Snowe of Maine and Chuck Hagel of Nebraska.

With a test vote set for Wednesday — capping a day and night of debate — Democratic officials conceded they were likely to get 52 or 53 votes at most. That's well short of the 60 needed to force a final vote on the measure.

While the issue was momentous — a war now in its fifth year costing more than 3,600 U.S. troops their lives — the proceedings were thick with politics.

MoveOn.org, the anti-war group, announced plans for more than 130 events around the country to coincide with the Senate debate, part of an effort to pressure Republicans into allowing a final vote on the legislation. A candlelight vigil and rally across the street from the Capitol was prominent among them, with House Speaker Nancy Pelosi, D-Calif., expected to attend.

Inside the Capitol, the session shaped up as the Senate's first all-nighter since 2003. Then, as now, the Senate staff wheeled about a dozen cots into a room near the chamber for any lawmakers needing them.

But the political roles were reversed. Four years ago, Republicans demanded votes on Bush's judicial nominees, and Democrats filibustered to avoid certain confirmation of several conservative appointees.

Then, Reid labeled the Republican-led all night-session a "circus," while other Democrats stoutly defended their right to set a 60-vote threshold for confirmation.

And then, McConnell talked critically of "unprecedented filibusters of President Bush's nominees" by Democrats, while other Republicans said they simply wanted an "up or down vote" on judicial appointments.

"Will the all-night session change any votes? I hope so," Reid said at midafternoon, pointedly stopping well short of a prediction that it would.

The maneuvering occurred as the Senate debated a broad defense bill that includes a pay raise for the troops, revised regulations for detaining suspects in the war on terror and an increase in the size of the Army and Marines.

Several officials said Reid might set the bill aside for weeks or months without completing it if, as expected, Republicans blocked a final vote on the withdrawal proposal.

In remarks on the Senate floor, Hagel took issue with fellow Republicans who said Bush's decision to increase troop strength, begun in January, deserves more time to work. "We must change our policy in Iraq," he said.

Smith and Snowe appeared with Democratic supporters of the legislation at a news conference.

"We are at the crossroads of hope and reality, and the time has come to address reality," said Snowe, who said the Iraqi government was guilty of "serial intransigence" when it came to trying to solve the country's political problems.

Smith, who is seeking re-election next year, said that Iraqis appeared focused on "revenge, not reconciliation" and that the administration needed to change its approach. "The American mission is to make sure that Iraq doesn't fall into the hands of al-Qaida," he said, rather than referee a civil war.

The legislation would require a troop withdrawal to begin within 120 days, to be completed by April 2008. The measure envisions leaving an undetermined number of troops behind, their mission limited to counterterrorism against al-Qaida and other groups, protecting U.S. assets and training Iraqi troops.

There are currently an estimated 158,000 U.S. personnel in Iraq, and supporters of the legislation have repeatedly declined to estimate how large a residual force they envision. "We're not going to get into numbers, because it changes the subject," said Sen. Carl Levin, D-Mich. The subject, he said, was focusing attention on Republican blocking tactics. Levin is chairman of the Senate Armed Services Committee and a leading sponsor of the measure along with Sen. Jack Reed, D-R.I.

While most Republicans have resisted the withdrawal bill, unhappiness with Bush's policy has been growing within the GOP ranks.

Sens. John Warner of Virginia and Richard Lugar of Indiana, two senior Republicans with long experience in military and foreign policy, last week proposed legislation to require Bush to submit a new strategy by Oct. 16. It would focus on protecting Iraqi borders, targeting terrorists, protecting U.S. assets and training Iraqi forces.

In addition, at least six Republicans support a bipartisan measure that would set a goal of beginning a troop withdrawal in early 2008.

In the complex political environment of the Senate, neither of those two measures seems likely to gain much traction in the next few weeks.

Democratic leaders oppose them as too weak to force a change in Bush's policy. Administration allies are determined to block any measure that contemplates a change in policy before September.