Three leading US senators have agreed on a resolution to oppose President George W Bush's plan to increase troop numbers in Iraq.
The senators want the Iraqi government to handle security
The group - two Democrats and a Republican - said the plan was not in America's interests, urging an early transfer of security to Iraqi leaders.
The move came as Democrats - who control Congress - began their own initiatives to block the deployment.
But the White House said Mr Bush would continue to proceed with his plan.
Under the new strategy announced last week, Mr Bush wants to send 21,500 additional troops to Iraq, the majority of them to violence-hit Baghdad, in an effort to improve security and end sectarian clashes.
The three senators behind the non-binding resolution are Democrats Joseph Biden and Carl Levin, and Republican Chuck Hagel, a long-standing critic of the war.
"It is not in the national interest of the United States to deepen its military involvement in Iraq," the resolution said.
"The United States should transfer, under an appropriately expedited timeline, responsibility for internal security and halting sectarian violence in Iraq to the government of Iraq and Iraqi security forces," it said.
Sen Biden said that the US presence in Iraq could only be sustained with support from Americans and the backing of Congress.
"Support is not there for the president's policy in Iraq," he said. "The sooner he recognises that reality and acts upon it, the better off all of us will be."
Another Republican senator, Olympia Snowe, later signed on to the resolution, which could be put to a vote within the next two weeks.
The BBC's James Coomarasamy in Washington says that the resolution would have no legislative effect, but would offer a way for like-minded senators to express opposition to Mr Bush's plan without having to take steps that could expose them to accusations of endangering American troops.
Democrats also stepped up efforts to thwart Mr Bush.
Sen Christopher Dodd introduced a bill that would require congressional approval for any troop increase in Iraq, while in the House of Representatives, a bill from three Democrats called for the withdrawal of US troops within six months.
Senator Clinton's remarks come as the US presidential race hots up
And a potential candidate for the Democratic presidential nomination, Sen Hillary Clinton, said she opposed the escalation and called for a cap on US troops in Iraq.
Instead, she said, US troops should be redeployed in Afghanistan. "Let's focus on Afghanistan and get it right," she said.
But the White House continues to argue that Congress has little room for manoeuvre since funds for the president's proposed troop increase have already been approved, our correspondent says.
'Get a lot worse'
Mr Bush, meanwhile, spent the day holding meetings with sceptical Republican lawmakers in a bid to shore up support for the deployment.
White House spokesman Tony Snow said the congressional resolutions would not affect Mr Bush's thinking.
"The president has obligations as a commander in chief," he said. "And he will go ahead and execute them."
On Tuesday, Mr Bush defended his plan in a televised interview, saying that the US had to up its engagement in Iraq so that the violence there did not "spiral out of control".
"If we don't help them stop, it's going to get a lot worse," he said.
In Iraq on Wednesday, at least 15 people were killed and more than 30 injured in a suicide car bomb attack at a busy market in the Shia district of Sadr City.
The attack came hours after at least seven policemen and civilians died when a lorry exploded near a police station in the northern city of Kirkuk.
On Tuesday, more than 70 people died in double bombings at a university in a Shia district in Baghdad.