Democrats in the US Congress have proposed legislation requiring all US troops to leave Iraq by August 2008.
The fate of US troops is causing political warfare in Washington
It is a direct challenge to President George W Bush, who has ruled out any fixed end to the Iraq operation.
Democrats, who took control of Congress in January, want to tie the measure to the $100bn (£52bn) Mr Bush has requested for Iraq and Afghanistan.
But White House staff said Mr Bush would veto the move, which they described as "a non-starter".
"What we're seeing here is an artificial, precipitous withdrawal from Iraq based on, unfortunately, politics in Washington, not conditions on the ground in Baghdad, Iraq," said presidential adviser Dan Bartlett.
Earlier on Thursday the top US general in Iraq said that the military alone could not provide a solution to the country's conflict.
Gen David Petraeus said it was critical that alienated groups be brought into talks. He also said he believed violence in Baghdad could be curtailed.
The Democrats' leader in the House of Representatives, Nancy Pelosi, said the bill would get US troops out of Iraq "safely, responsibly and soon".
"It calls for the strategic redeployment of US combat troops some time in 2008. Only then can we refocus our military efforts on Afghanistan," she said.
The bill calls for the redeployment to begin as early as July this year unless Mr Bush can certify that progress is being made in bringing order to the violence-wracked country. At the latest, it would start in March 2008 and be completed within six months.
'Tying generals' hands'
The BBC's Washington correspondent James Coomarasamy says the bill is the latest attempt by the Democratic leadership to unite a party which remains divided on how to move forward in Iraq.
Some on the anti-war wing of the party say the bill does not go far or fast enough.
Republicans, meanwhile, have called the proposal a vote to tie the hands of US generals in Iraq.
House Republican Leader John Boehner said the measure amounted to "establishing and telegraphing to our enemy a timetable" and would lead to failure in Iraq.
"[US commander in Iraq] General Petraeus should be the one making the decisions on what happens on the ground in Iraq," he added.
At his first news conference since taking command last month, Gen Petraeus spoke of the need for political engagement to help ease the unrest in Iraq.
"Military action is necessary to help improve security... but it is not sufficient. There needs to be a political aspect," he said, adding that some groups "who have felt the new Iraq did not have a place for them" would have to be engaged in talks.
He was speaking after the US defence secretary approved an extra 2,200 military police to aid a security crackdown in Baghdad.
The new Baghdad offensive involves US and Iraqi forces, thousands of whom are already on the ground, sweeping the city for militants and illegally held weapons.
Gen Petraeus said: "It's too early to discern significant trends, but there have been a few encouraging signs." However he admitted "sensational attacks inevitably will continue".
BBC defence and security correspondent Rob Watson says that despite the scale of the new Baghdad drive, there simply are not enough US troops to prevent the violence shifting to other areas.
Our correspondent says that privately US officials believe it will not be possible to judge whether the surge has worked until all the troops have arrived in the summer and, if it does not, there will be few options remaining.