The newly-empowered US Democrats have warned President George W Bush he will have to justify any plans to boost troop levels in Iraq.
Mr Bush could announce up to 20,000 more troops for Baghdad
House leader Nancy Pelosi said there would be "no blank cheque" for Mr Bush.
The president is expected to outline a new strategy this week that may include 20,000 more US troops for Baghdad.
Reports suggest Mr Bush intends to attach strong conditions to any increase, such as specific targets for Iraq's security and political progress.
The Democrats took over Congress for the first time in 12 years last week, giving them a shift in power which could lead to challenges to the authority of the White House.
Ms Pelosi described the conflict in Iraq as a war without end which the American people had rejected.
She told CBS News programme Face the Nation that up until now the Republicans had given Mr Bush a blank cheque, with no oversight, standards or conditions.
"Congress is ready to use its constitutional authority of oversight to question what is the justification for this spending, what are the results we are receiving," she said.
"Escalation of the war is opposed by the Democrats."
Ms Pelosi has urged a phased withdrawal of US troops
However, Ms Pelosi said the Democrats would not cut off money for troops already in Iraq.
Another top Democrat, Steny Hoyer, told another US television show the expected proposal by Mr Bush for a $1bn boost for the Iraq economy would also "get careful scrutiny and oversight".
Some Republicans have said there should be no Congress block on financing.
Senate Minority Leader Mitch McConnell said: "Congress is incapable of micromanaging the tactics in the war."
And Democratic Senator Joe Biden added: "As a practical matter, there's no way to say, 'Mr President, stop'."
Ms Pelosi and Senate majority leader Harry Reid sent a letter to Mr Bush last week saying there should be no increase in troops, but instead there should be the start of a phased withdrawal.
Mr Bush is continuing briefings with lawmakers this week ahead of a speech expected as early as Wednesday.
The expected policy shift follows a recent report from the Iraq Study Group, which said US policy in Iraq was not working, and a Republican bruising in mid-term elections in November, blamed partly on the White House's handling of the conflict.