Even if those proposals draw the 60 votes needed to overcome a Senate filibuster — a level that has eluded Democrats this year — any real strictures on the president would face a veto, frustrating war critics and raising the prospect that roughly as many American troops might be in Iraq a year from now as were there a year ago.

Still, the Democrats tried to get ahead of President Bush's planned speech on Iraq on Thursday night, and to press what they see as a political advantage in opposing the war in the months before the 2008 elections.

Senator Harry Reid of Nevada, the majority leader, and two party leaders on military issues accused Mr. Bush of embracing "more of the same" and of trying to pass off a routine troop reduction as a significant shift in policy.

"That is unacceptable to me, it is unacceptable to the American people," said Mr. Reid, who was flanked by Senators Carl Levin of Michigan, chairman of the Armed Services Committee, and Jack Reed of Rhode Island, a West Point graduate.

Democratic presidential contenders also assailed the administration's plan.

Senator Barack Obama of Illinois called for the withdrawal of one or two combat brigades a month, starting immediately.

Senator Hillary Rodham Clinton of New York said taking credit for the force reductions that Gen. David H. Petraeus, the commander in Iraq, was recommending, and that Mr. Bush appeared ready to accept, was "like taking credit for the sun coming up in the morning."

With Democrats intensifying their attacks on the strategy outlined this week by General Petraeus, the administration is setting in motion its plans with a prime-time speech by the president on Thursday, a subsequent visit to a military base and continued appearances by General Petraeus and Ryan C. Crocker, the American ambassador in Baghdad.

At a news conference on Wednesday, General Petraeus reiterated that he was unwilling to commit to troop cuts beyond a five-brigade reduction by mid-July, a level he described as prudent. There are 20 combat brigades in Iraq.

He also took issue with claims that such a reduction would not be significantly faster than what had already been scheduled. Combat forces in Iraq serve up to 15-month tours. Under that limit, part of the Pentagon's broad effort to lessen the strains on the military, General Petraeus would not have had to pull out any combat units until April, instead of removing the first brigades in mid-December, he said.

"We are coming out quicker than we had to," he said.

The White House spokesman, Tony Snow, in his last news briefing before leaving the job, rejected Democrats' complaints that the administration's plan was simply a return to the level that existed before more than 30,000 additional troops were sent into Iraq this year, a buildup that the administration pointedly referred to as a surge, suggesting its temporary nature.

"It's a different country," said Mr. Snow of Iraq. "You have the ability to reduce the numbers because there have been changes that reduced the necessity of American involvement."

Senator Reid would not provide details of the legislative proposals that Democrats will pursue. But Mr. Levin and Mr. Reed have been working with some Republicans on a measure that would focus the military mission on counterterrorism, training Iraqis and protecting forces already there — a switch intended to allow large numbers of combat troops to be withdrawn by next spring.

"We have to go ahead and recognize the strain on the military forces and give them the tasks that they can do so well," said Mr. Reed, a former Army captain, "but within the capability of their resources and the best interests of the United States."

They have been exploring the idea of making the withdrawal more of an objective than a requirement in order to attract Republican votes, but that approach could cause defections by Democrats.

Democrats have been picking up new Republican support for a measure that requires troops to spend at least the same amount of time at their home bases as they did in Iraq before returning — a requirement that could reduce troop numbers because the Pentagon would not have as many eligible for deployment.

"I think that might be a good way to accelerate a troop reduction," said Senator Gordon Smith, Republican of Oregon, who noted that it was also popular with strained military families.

That measure attracted 56 votes this summer, and some Republicans who opposed it then, including George V. Voinovich, Republican of Ohio, have expressed new interest. Senator Arlen Specter, Republican of Pennsylvania, said Wednesday that he was considering the proposal, and Democrats were also trying to persuade Senator Lisa Murkowski, Republican of Alaska.

Mr. Reid said Democrats also planned to vote on more aggressive legislative challenges to the war, which could help appease critics who are demanding that Democrats take tougher action.

Democrats say they may also be more willing to try to attach conditions to coming Pentagon spending requests. (Democrats have been reluctant to limit money for the war unilaterally.) "I think the American people are getting tired of sending the money with no end in sight," said Senator Charles E. Schumer, Democrat of New York.

The struggle to settle on a party alternative illustrates the problems Democrats are having finding a way to take on the president that unites their party and avoids criticism that they are weak on national security.

As Democrats huddled Wednesday to prepare for the floor debate, a group of leading House Republicans arrived in Iraq to demonstrate their backing for the president. The lawmakers, led by Representative John A. Boehner of Ohio, the Republican leader, had been in Iraq less than five hours, but in a conference call with reporters they said their initial briefings had already confirmed improvements.

"Clearly what's happened over the last three months has been real success," said Mr. Boehner, who previously visited Iraq in July 2006.

In an interview on "The Today Show" on Wednesday, Secretary of State Condoleezza Rice said stabilizing Iraq was part of "a long process of dealing with what the president called a long time ago a generational challenge to our security brought on by extremism, coming principally out of the Middle East."

Speaker of the House Nancy Pelosi, a California Democrat, said Ms. Rice's comment represented an acknowledgment that the United States would be engaged in Iraq for "years to come."

"We need a new direction that redeploys our troops from Iraq, rebuilds our military and refocuses on fighting terrorism across the world," Ms. Pelosi said.