WASHINGTON — Raising the stakes in her bid to stop the war, House Speaker Nancy Pelosi today is expected to circulate a draft bill to her colleagues setting binding deadlines for the withdrawal of American forces from Iraq.

At a press conference yesterday, Ms. Pelosi, a Democrat of California, promised that the troops fighting in Iraq would be home by August 2008, a time frame that coincides with the homestretch of the presidential contest that year. Her position echoes that of the three leading Democratic contenders for the nomination.

The proposal would tie a withdrawal date for soldiers to specific timelines of 180 days if key deadlines — such as the repeal of a debaathification law and the passage of a plan to share the country’s oil revenues — are not met. Unlike earlier Democratic proposals against the troop surge, the new plan, to be a part of the House budget bill funding the $100 billion the White House has requested for the surge of 21,500 soldiers to Baghdad and Anbar province, would be binding law.

On the Senate side, Majority Leader Harry Reid, a Democrat from Nevada, introduced a similar, binding withdrawal proposal that he asked his Republican counterpart’s consent to introduce next week as a stand-alone bill. Senator McConnell, a Republican of Kentucky, declined to endorse the proposal, which would begin the redeployment of American troops by March 31.

“Unless there is progress made in meeting benchmarks by July 2007, we begin the redeployment of our troops out of a combat role in Iraq,” Ms. Pelosi said.

According to an aide in the House leadership, the bill would include four exemptions for when troops in Iraq could stay even if deadlines and benchmarks are not met. They include troops for training Iraqi police and soldiers, targeted counterterrorism, troop protection, and the protection of diplomats. The list of exemptions pointedly does not include the protection and garrisoning of city blocks in Baghdad, a function tasked to the bulk of the 21,500 soldiers the president has sent and is sending to Iraq’s capital.

“The president would only be authorized to spend money to redeploy, or to deploy under the exceptions,” the source, who requested anonymity because the House Democratic caucus had yet to see the proposal, said. “Any other use would be against the law, and the sanctions available whenever a law is violated would then be available.”

The language on troop withdrawal is part of a spending bill, and the proposal is binding, which means the House leadership is edging closer to embracing the funding cut-off they pledged they would not endorse in December.

On December 5, Ms. Pelosi told reporters, “We will not cut off funding for the troops. … Absolutely not.” While she said Congress would practice oversight over budget requests for the war, she distinguished herself from the left flank of her party. The setting of a binding deadline is also a new position for the speaker, who opposed a nonbinding deadline resolution last year when it was broached by Rep. John Murtha, a Democrat of Pennsylvania.

Even the plan unveiled yesterday by the House speaker, however, has drawn criticism from the doves. A counterproposal to set the withdrawal deadline at December 31 of this year, put forward by the only House representative to vote against the authorization of force against the Taliban, Rep. Barbara Lee, a Democrat of California, has attracted support from the “out of Iraq caucus.”

The White House yesterday was quick to threaten a veto of any House spending measure that would set time lines and benchmarks. Speaking to reporters en route to Latin America, a counselor to the president, Dan Bartlett, called the proposal “a political compromise in the Democratic caucus of the House aimed at bringing comity to their internal politics, not reflective of the conditions on the ground in Iraq.” He added, “It would unnecessarily handcuff our generals on the ground, and it’s safe to say it’s a nonstarter for the president.”

The House minority leader, John Boehner, called Ms. Pelosi’s proposal a “road map for terrorists.”

Yesterday, the top American military commander in Iraq, General David Petraeus, told reporters in Baghdad that 40 garrisons inside Iraqi neighborhoods have already been established and that some progress has been made in the surge. But the general also cautioned that the new surge would “take months, not days or weeks, to fully implement.”

A spokesman for Mr. Boehner, Brian Kennedy, said he expected that the Democratic proposal could unify the Republican conference further and might even force more conservative Democrats to vote with the House minority. “Each new plan they come out with is a little more outrageous than the last one,” he said. “All of them have served to unify the Republican conference in the House, and they seem to be pushing a good number of moderate Democrats in our direction, as well.” Just two Democrats voted against the nonbinding resolution last month opposing the surge to Baghdad.

A spokesman for Senator Reid, Jim Manley, called the new emphasis on binding legislation requiring troop withdrawal a new strategy, after efforts in the Senate to break a filibuster of a nonbinding resolution failed.

“This is the next phase of the strategy designed to hold the president accountable,” Mr. Manley said. “It is binding legislation to force the president to change course on Iraq.”