WASHINGTON — House members debated Iraq yesterday in an emotional and historic face-off over a war that the House speaker, Rep. Nancy Pelosi, condemned as a commitment with “no end in sight.”

The long-awaited floor debate came with Democrats now as the majority party in Congress, the war nearly four years old and more than 3,100 Americans dead. Lawmakers argued about whether to publicly rebuke President Bush for sending 21,500 more troops into battle.

“The American people have lost faith in President Bush’s course of action in Iraq, and they are demanding a new direction,” said Ms. Pelosi, a Democrat of California. Her counterparts among the Democratic leaders in the Senate worked to bring their own measure to the floor.

A resolution putting the House on record against Mr. Bush’s expansion of troop strength was expected to be approved by week’s end. It was nonbinding but nevertheless unmistakable in its message. “No more blank checks for President Bush on Iraq,” Ms. Pelosi declared.

Countered Bush press secretary Tony Snow at the White House: “Members of the House and members of the Senate have the freedom to go ahead and write their resolutions and do what they want with them. The one thing we do expect is, we do expect those who say they’re going to support the troops, to support them.”

Republicans, the minority party on the Hill for the first time in 12 years, issued impassioned warnings of the consequences of undermining the president’s policies in Iraq. “We will embolden terrorists in every corner in the world. We will give Iran free access to the Middle East,” said the minority leader, Rep. John Boehner, a Republican of Ohio. “And who doesn’t believe the terrorists will just follow our troops home?”

Mr. Boehner teared up before reporters as he listened to Rep. Sam Johnson, a Republican of Texas, describe being a prisoner of war in Vietnam and learning of American protests back home.

The House majority leader, Rep. Steny Hoyer, a Democrat of Maryland, insisted the Democrats had no intention of impeding the mission of those in Iraq. “There will be no defunding of troops in the field. There will be no defunding which will cause any risk to the troops,” he told a news conference.

The House rejected, on a 227–197 vote, a Republican procedural attempt to force a vote on a proposal that would have barred Congress from cutting off funding for American troops in harm’s way.

Democrats expressed confidence their measure would prevail and said they would attempt to use it as the opening move in a campaign to pressure Mr. Bush to change course and end America’s military involvement in the war.

In the Senate, the majority leader, Senator Reid of Nevada, said he has begun the process of bringing the House resolution up for a vote in his chamber in two weeks, on February 27.

Mr. Reid said yesterday the House language would be a model for the Senate version. “We support the troops, oppose the surge, perfect,” Mr. Reid said, summarizing the House resolution.

In the House, Democrats called on several freshmen who served in the military to make their argument against further commitments in Iraq.

Rep. Patrick Murphy, a Democrat of Pennsylvania, a captain in the Army’s 82nd Airborne, said that “three years after I left Iraq, Americans are still running convoys up and down Ambush Alley and securing Iraqi street corners.”

But Rep. David Dreier, a Republican of California, stressed that “we go to war to win, we go to war with a mission.” He said, “We dishonor the lives of those who have made the ultimate sacrifice if we, in fact, abandon that mission. … We have a duty to pursue nothing less than victory.”

Republicans conceded that the measure was headed for approval and said a few dozen members of the GOP were likely to vote for it.

It was the first debate about the war in either house of Congress since November’s midterm elections, when public opposition to the conflict helped power Democrats to control of the House and Senate.

Mr. Bush’s decision last month to deploy an additional 21,500 troops to help stop sectarian violence has quickly become a flashpoint for critics of the war in Congress. There are currently about 141,000 American troops in Iraq and 27,000 in Afghanistan.

The nonbinding measure states simply that the House “will continue to support and protect” troops serving in Iraq but that it “disapproves” of the troop buildup.

While such legislation can neither force Mr. Bush’s hand nor bring the war to a close, the vote could be a politically embarrassing rejection of his Iraq policy and help Democrats reassert congressional oversight of the war.

Each of the House’s 435 members and five delegates were being allotted five minutes to speak on the issue. Democratic leaders said Monday they planned to restrict members to a single vote by week’s end, barring any amendments or a GOP alternative — a tack Republican leaders decried as unfair.

“After promising to make this Congress the most open and honest in history, Nancy Pelosi has effectively shut out both Republicans and Democrats from substantively debating the most important issue of our time,” said Rep. John Shadegg, a Republican of Arizona.

Ms. Pelosi and other Democrats said restricting debate to one measure would force members to go on record on the war without hiding behind political ploys.

This week’s debate will be in sharp contrast to the one in 2002, which authorized Mr. Bush to use force if Saddam Hussein did not comply with U.N. weapons inspectors. That debate resulted in solid margins of support from Republicans and Democrats.

In October 2002, just over half of the public — 52% — approved of Mr. Bush’s handling of Iraq in Gallup polling. Mr. Bush now faces a new political landscape, and the invasion has been discredited with a majority of the public.