In a direct challenge to President Bush, the House of Representatives has, on a roughly party line vote of 218 to 212, called for the withdrawal of U.S. troops from Iraq no later than the end of August 2008.
The president has condemned the House action, and reiterated a veto threat.
Democrats succeeded in passing a measure that ties funding needed to support U.S. troops in Iraq and Afghanistan with requirements to limit the duration of deployments, and force President Bush to certify the readiness of military units.
The beginning of a withdrawal period would be triggered, if the president cannot certify to Congress at two key points this year, that the Iraqi government is satisfactorily reaching political, economic and security benchmarks.
During the debate, the Democratic appropriations chairman, David Obey, asserted that, wording, timelines and benchmarks aside, the primary significance is Congress' determination to exert some control over the president's Iraq policy.
"What is important is that, for the first time, this Congress will be exercising its constitutional responsibilities to provide real oversight on the executive branch of government, and we will be trying to set this country on a new direction."
Congressman John Murtha, a Vietnam war veteran, referred to U.S. troops killed and wounded in Iraq, and money in the bill for military health care, saying Iraqi leaders must be held to commitments to shoulder more of the burden.
"When you see three-thousand U.S. troops that have been killed, or 25-thousand that have been wounded, it is individuals that have been wounded, individuals that have been killed, and those families are suffering. They have a civil war. We have to put benchmarks in this bill, so the Iraqis start to do it themselves, and the Americans aren't forced to make up the difference and do it themselves."
Democratic leaders were generally successful in persuading most outspoken anti-war Democrats to support the bill, even though it gives a longer timetable for withdrawal than they wanted.
Only 14 crossed party lines to oppose their leadership, while only two Republicans voted for the bill.
Republican minority leader John Boehner made a final appeal against the measure.
"Who doesn't believe that if we go down this path, we are going to leave chaos and genocide in Iraq, and we are going to tell our enemies all around the world that you can take on the United States, you can push them to the edge, and, at the end of the day, they will just go home?"
Both sides also turned to war veterans to make their case. Sam Johnson is a Texas Republican.
"Internationally announcing our timelines for withdrawal literally hands the enemy our war plan, and gives them hope that they will win, if they just wait it out. What world superpower would do such a thing?"
Pennsylvania Congressman and Iraq war veteran Patrick Murphy delivered emotional remarks for Democrats.
"In the last four years the Republican Congress followed this president, as thousands of brave American soldiers returned home in coffins with our American flag. Nineteen of those coffins had American soldiers I served with in Iraq, 19 paratroopers."
House Speaker Nancy Pelosi spoke to reporters after the vote.
"Congress voted 'no' to giving a blank check to an open-ended commitment to war without end to the President of the United States, and 'yes' to begin the end of the war and the redeployment of our troops."
The Senate, which Democrats narrowly control, may begin debate next week on a measure containing about three-billion-dollars less than the House version, with a non-binding call for a U.S. withdrawal by March 2008.
But any legislation containing a timetable or other conditions faces a veto threat from President Bush, who condemned the House action as, in his words, "an act of political theater."
"Amid the real challenges in Iraq, we're beginning to see some signs of progress. Yet, to score political points, the Democratic majority in the House has shown it is willing to undermine the gains our troops are making on the ground."
House Speaker Pelosi declines to say what strategy House Democrats will take in negotiations with the Senate, should a bill pass there, to reconcile differences to avert a veto.
House Republicans assert Democrats will ultimately be forced to accept a clean bill, without troop readiness requirements or withdrawal timetables, that the president will accept.