WASHINGTON, March 10 — Nancy Pelosi, the House speaker, challenged President Bush on Saturday over his threat to reject an Iraq spending bill if it calls for a troop withdrawal, even as the administration sought to shift money to pay for additional forces in Iraq and Afghanistan.

With a House committee set to consider the approximately $100 billion measure as early as Thursday, Ms. Pelosi said a veto would suggest to Iraqi leaders that the United States was not serious about making them more responsible for policing their own country.

"With his veto threat," she said in a statement, "the president offers only an open-ended commitment to a war without end that dangerously ignores the repeated warnings of military leaders, including the commander in Iraq, General Petraeus, who declared in Baghdad this week that the conflict cannot be resolved militarily." In his comments, Gen. David H. Petraeus spoke about the long-term challenges facing the troops in Iraq.

Traveling in Latin America, Mr. Bush sent the speaker a request Saturday to adjust the administration's spending proposal by shifting $3.2 billion from "lower priority" programs to pay for about 4,400 troops to bolster the 21,500 increase already sought for Iraq. The added forces would be split between combat and support units.

The new request also seeks more than $500 million to send additional combat troops, linguists and military trainers to Afghanistan, according to the Office of Management and Budget, "in anticipation of increased combat operations against the resurgent Taliban."

"This revised request would better align resources based on the assessment of military commanders to achieve the goal of establishing Iraq and Afghanistan as democratic and secure nations that are free of terrorism," Mr. Bush said in his letter to Ms. Pelosi.

The request for additional troops is likely to figure into the coming debate over war spending. Nearly all Democrats oppose the buildup and some have complained that the White House did not factor the needed support forces into its initial call for 21,500 troops.

A spokesman for Mr. Bush said this week that the president would reject the legislation if Democrats followed through with their plan to require most American forces to be out of Iraq by September 2008, or earlier, if Iraq does not show progress in securing its territory.

Trying to build backing for the plan, Ms. Pelosi and her fellow leaders are drafting a proposal that can satisfy both moderate Democrats worried about a precipitous withdrawal and party members who want to spend money only on a pull-out. The emerging legislation will also have money for military health care and unrelated provisions that can attract votes.

Commentary: