"If the president wants to add to this mission, he is going to have to justify it," she said. "And this is new for him because up until now the Republican congress has given him a blank cheque with no oversight, no standards, no conditions."
Bush, who is reshuffling his Iraq team, is expected to outline a shift in course in the unpopular war this week with what could be a short-term increase of up to 20,000 US troops to try to restore stability to Baghdad.
Pelosi and Harry Reid, the Democrat senate majority leader from Nevada, urged Bush in a letter last week to reject a troop increase and begin a phased withdrawal from Iraq.
On Sunday, Mowaffaq al-Rubaie, the Iraq national security adviser, said that the Iraqi government would accept higher US troop levels. "We will support them," he said.
Joseph Biden, the Delaware Democrat senator who chairs the senate foreign relations committee, said he has drafted a senate "resolution of disapproval" to try to dissuade Bush from seeking higher troop levels.
But Biden also conceded that congress can do little to prevent Bush from acting as he sees fit.
"It's a sense of the senate to try to convince the president that there are significant numbers in the United States senate who think this proposal is a mistake and hopefully force him to reconsider," he said.
"This is a prescription for another tragedy. There is now a civil war. You need a political solution."