Yet Pelosi's second-in-command and a Senate leader on foreign affairs questioned the wisdom and legality of using the power of the purse to thwart the White House as Bush prepared to announce his revised war strategy this week - perhaps on Wednesday.
Republicans, now in the minority, said more troops were needed to get a handle on the spiraling violence in Iraq. They also cast doubt whether Democrats would - or could - block the president's plans.
"Congress is incapable of micromanaging the tactics in the war,"said Senate Minority Leader Mitch McConnell, R-Ky.
Pelosi made clear that her party supported boosting the overall size of the military "to protect the American people against any threats to our interests, wherever they may occur. That's different, though, from adding troops to Iraq." She also said Democrats would not cut off money for those troops already in Iraq.
"The burden is on the president to justify any additional resources for a mission," said Pelosi, D-Calif. "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 check for him to do whatever he wishes there," she added in an interview taped Saturday and broadcast Sunday.
Asked about Pelosi's remarks, White House spokesman Alex Conant said Bush welcomed any ideas on Iraq that "lead to success."
"We're glad the speaker wants us to succeed in Iraq," he said.
While leading Democrats reaffirmed their opposition to a troop buildup, several did not join Pelosi in suggesting it was possible Congress could deny Bush the money for the additional forces.
"I don't want to anticipate that," said House Majority Leader Steny Hoyer, D-Md.
Sen. Joe Biden, chairman of the Senate Foreign Relations Committee and a 2008 presidential candidate, said increasing troops would be a "tragic mistake." But he contended Congress was constitutionally powerless to second-guess Bush's military strategy because lawmakers had voted to authorize the commander in chief to wage war.
"As a practical matter, there's no way to say, 'Mr. President, stop,"' Biden said, unless enough congressional Republicans join Democrats in persuading Bush that the strategy is wrong. "You can't go in and, like a tinker toy, and play around and say, 'You can't spend the money on this piece and this piece."'
Sen. Lindsey Graham, R-S.C., criticized the Democratic proposal as a "formula for defeat," saying more troops are needed because the U.S. military never had enough personnel in Iraq to get the job done.
Sen. John McCain, R-Ariz., wrote in Sunday's Washington Post that boosting troops for an indefinite time was necessary to secure peace in the Mideast.
"When we authorized this war, we accepted the responsibility to make sure they could prevail," he wrote. "Even greater than the costs incurred thus far and in the future are the catastrophic consequences that would ensure from our failure in Iraq."
But Pelosi, pointing to the November elections that ousted Republicans from control of the House and Senate, said Iraq already is in complete chaos.
"The American people have spoken very clearly on the subject in the election," she said. "And this war in Iraq is damaging our military readiness, so it is not making America safer, it is not making the region more stable."
Sen. Barbara Boxer, D-Calif., a member of the Senate Foreign Relations Committee, said she would like to see a congressional vote on whether Bush can send in additional troops.
"My belief is, the president's coming to us," she said. "He's going to ask for billions and billions of dollars. He's going to send more of our people into harm's way. I think it would be best for the country if we got to vote on that surge or escalation."