New Democrat House Speaker Nancy Pelosi has warned President Bush to think twice before proposing a troop increase in Iraq.
Mr Bush was putting the finishing touches on a revised war strategy he planned to announce this week. In addition to a troop increase, the proposal could provide more money for jobs and reconstruction programmes in Iraq.
US House Speaker Nancy Pelosi
In issuing her warning, Ms Pelosi made clear that her party supported boosting the overall military size "to protect the American people against any threats to our interests" and would not cut off money for troops already in Iraq.
But Mr Bush will not get a blank cheque for an open-ended commitment there, she said.
Any funding he seeks for additional forces in Iraq - Mr Bush's expected plan could send as many as 20,000 more US troops - will get the "harshest scrutiny," she added.
She said: "The burden is on the president to justify any additional resources for a mission. 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.
"There's not a carte blanche, a blank cheque for him to do whatever he wishes there," she added.
Asked about Ms Pelosi's remarks, a White House spokesman said Mr Bush welcomed any ideas on Iraq that "lead to success."
Since the attacks of September 11th, 2001, Congress has approved about $500 billion for Iraq, Afghanistan and other terrorism-fighting efforts.
The White House is working on its largest-ever appeal for more war funds - a record $100 billion. It will be submitted along with Mr Bush's February 5th budget.