WASHINGTON —  Democrats relentlessly assailed President Bush's policy in Iraq as a catastrophic failure Tuesday as the House plunged into momentous debate on a war that has lost public support and cost more than 3,100 U.S. troops their lives. "No more blank checks," declared Speaker Nancy Pelosi.

"This battle is the most visible part of a global war" against terrorists, countered the Republican leader, Rep. John Boehner, hoping to limit GOP defections on what loomed as an extraordinary wartime rebuke to the commander in chief. "If we leave, they will follow us home. It's that simple."

The Democratic leadership set aside most of the week for the historic debate, expected to culminate in a vote on Friday on a bare-bones, nonbinding resolution that "disapproves of the decision of President George W. Bush ... to deploy more than 20,000 additional United States combat troops to Iraq."

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The 95-word measure adds that "Congress and the American people will continue to support and protect the members of the United States armed forces who are serving or who have served bravely and honorably in Iraq."

The debate was Congress' first on Iraq since Democrats gained control of the House and Senate in midterm elections shadowed by voter opposition to the war. Decorum carried the day in the chamber — where catcalls are part of near-daily discourse — as Democrats and Republicans took their five-minute speaking turns across the hours.

Passage was a virtually certainty. Democratic leaders said they expected no more than one or two members of their rank-and-file to oppose the resolution. Republicans said that despite quiet lobbying by the White House, they expected at least two dozen GOP lawmakers to swing behind the measure, suggesting that it would command the votes of at least 250 or 260 votes in the 435-member House.

"A vote of disapproval will set the stage for additional Iraq legislation, which will be coming to the House floor," said Speaker Pelosi of California, who underscored the significance of the debate by delivering the first speech.

"In a few weeks, the war in Iraq will enter its fifth year, causing thousands of deaths, tens of thousands of casualties, costing hundreds of billions of dollars and damaging the standing of the United States in the international community. And there is no end in sight," she said.

Boehner followed her to the well of the House seconds later, the first Republican to speak.

"There is no question that the war in Iraq has been difficult. All Americans are frustrated we haven't seen more success more quickly," he conceded. Pivoting quickly, he called the Iraq War the latest in a string of conflicts dating to the founding of the nation more than two centuries ago.

"Every drop of blood that has been spilt in defense of freedom and liberty — from the American Revolution to this very moment — is for nothing if we are unwilling to stand against this threat," he said.

Republican congressional aides said the White House was working against the measure, although presidential press secretary Tony Snow, asked if that was the case, said "no."

"We've made our views known, in terms of what people have to keep in mind. But 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," he said.

Additionally, ambassadors from Saudi Arabia, Egypt, Jordan and Qatar met with several Republican lawmakers during the day and warned them of the consequences of a precipitous U.S. withdrawal from Iraq. Rep. Todd Akin, R-Mo., said one ambassador compared the U.S. involvement in Iraq to open-heart surgery — requiring the surgeon to stay until the job was finished.

One by one, Democrats cast the war in starkly different terms.

"The administration's policy on Iraq has failed. It failed yesterday, it's failing today, and it will fail tomorrow," said Rep. Peter Welch of Vermont, serving his first term in Congress after winning his seat last fall. "These failures have left America weakened, not strengthened."

Rep. John Dingell of Michigan, who served in World War II and has been in Congress since 1955, joined the chorus of critics. "When faced with a choice of approving of the president's policy or giving a vote of no confidence, the choice is easy," he said. "I cannot support, nor will I condone, any policy that continues the long train of failure that brought us to this point."

Republican supporters of the administration countered, but were urged to do so carefully.

"If we let Democrats force us into a debate on the surge (in troops) or the current situation in Iraq, we lose," Reps. John Shadegg of Arizona and Pete Hoekstra of Michigan said in a letter to fellow Republicans.

"As in the Cold War, our current struggle is one of survival," Rep. Ileana Ros-Lehtinen, R-Fla., said in floor debate. "The enemy does not mean merely to chase us away. The goal of the Islamist extremist radicals is to destroy us. If we run, they will pursue. If we cower, they will strike."

"The world is watching. The radical jihadists who oppose us are watching," said Shadegg, warning against anything that could signal weakness on the part of the United States.

Republicans had sought to offer an alternative measure, drafted by Rep. Sam Johnson, R-Texas, that would have prohibited Congress from cutting off funds for the troops. Johnson was a prisoner of war during Vietnam, and Boehner teared up before reporters as he listened to him describe his reaction at the time when he learned of anti-war protests back in the United States.

But Democrats said Republicans would not be allowed a vote on their measure, and the House voted, 227-197, to uphold the rejection.

Majority Leader Steny Hoyer, D-Md., said Democrats have no intention of cutting off funds for troops in the field. "There will be no defunding which will cause any risk to the troops," he told a news conference.

Numerous Democrats have expressed a determination to withdraw combat forces from Iraq, but they also say they would do so in a way that did not expose the troops to additional danger.

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