US Senator Edward Kennedy has launched a bid to prevent President George W Bush sending more troops to Iraq.
Sen Kennedy has been a long-time opponent of the Iraq war
The veteran Democratic Party senator said he would propose legislation requiring congressional approval for any further deployment of US troops.
Committing more troops to Iraq would be "an immense new mistake", he said.
Mr Bush is expected to set out his new strategy for Iraq on Wednesday - a strategy that could include an increase of up to 20,000 troops.
Senator Kennedy - a long-standing opponent of the war - said Democrats had to act to prevent an escalation of troops in Iraq.
"The best immediate way to support our troops is by refusing to inject more and more of them into the cauldron of a civil war that can be resolved only by the people and government of Iraq," he said.
The American people had sent a clear message in the November mid-terms that they wanted a change of course in Iraq, he said.
"President Bush should not be permitted to escalate the war further, and send an even larger number of our troops into harm's way, without a clear and specific new authorisation from Congress."
Democrats control Congress for the first time in 12 years and House leader Nancy Pelosi warned Mr Bush on Monday that he would have to justify any plans to boost troop levels in Iraq.
Senator Kennedy's comments came as a poll showed that 61% of Americans opposed a troop increase.
Approval of Mr Bush's handling of Iraq stood at a new low of 26%, the USA Today/Gallup poll showed.
The BBC's Justin Webb in Washington says that Senator Kennedy's effort will get a far better hearing in Congress than it would have done when the Republicans were in charge but it faces significant hurdles.
Many Democrats will feel queasy about voting to interfere in military matters, particularly if the newly-appointed commanders in Iraq say they need the reinforcements, our correspondent says.
Still, Senator Kennedy feels that most Americans are coming round to his way of thinking and he may well be right, he says.
Mr Bush is due to speak in Washington at 2100 local time on Wednesday (0200 GMT Thursday).