WASHINGTON -- House Speaker Nancy Pelosi and Senate Majority Leader Harry M. Reid yesterday declared that "it is time to bring the war to a close," and warned President Bush that sending more U.S. troops to Iraq would be unacceptable to the Democratic majorities that have just taken over Congress.

Directly challenging Mr. Bush's wartime leadership on their second day in charge on Capitol Hill, the two Democratic leaders, Ms. Pelosi of California and Mr. Reid of Nevada, sent Mr. Bush a letter suggesting that, instead of a short-term escalation, he begin a phased withdrawal of U.S. forces in the next four to six months. The mission of remaining troops, they said, should be shifted away from combat toward more training, logistics and counterterrorism.

The newly ascendant Democrats are trying to pre-empt the president even before he announces his new strategy. As he prepares for a nationally televised address next week, officials said Mr. Bush is considering three main options to bolster U.S. forces in Iraq -- a relatively modest deployment of fewer than 4,000 additional troops, a middle-ground alternative involving about 9,000 and the most aggressive adding 20,000 more troops.

In a speech today unveiling his own revised security plan, Iraqi Prime Minister Nouri al Maliki is expected to publicly welcome additional U.S. troops, a condition requested by the Bush administration. Mr. Maliki's cooperation is pivotal to Mr. Bush's own efforts. Mr. Bush told Mr. Maliki in their video-conference discussion Thursday that the United States is willing to help, but that Mr. Maliki must deliver along the way, U.S. officials said.

In preparation for the shift in strategy, Mr. Bush reshuffled his national security leadership team yesterday. He replaced the top two generals running the Iraq war, named a new Army chief of staff, moved his intelligence director over to the State Department and put a veteran officer in charge of intelligence. Officials have said he also plans to move his ambassador in Baghdad to the United Nations and replace him with a veteran diplomat.

Over the next few days before his speech, Mr. Bush is conducting intensive consultations with lawmakers, foreign allies and advisers. He met with lawmakers from both parties yesterday and, according to U.S. officials and diplomats, he plans to talk with leaders of Britain, Australia and possibly Denmark -- countries that still have major military contingents in Iraq. The White House said he will talk with both Ms. Pelosi and Mr. Reid before announcing his new strategy.

White House press secretary Tony Snow said the sessions with lawmakers have featured "some vigorous exchanges," but have been useful. "The fact is that these meetings may not be happy face-kumbaya, but they have been very constructive in the sense that people are talking respectfully about important issues and expressing their ideas," he said. "And some of them are quite interesting. And we're taking them into account."

But some lawmakers have left meetings unsatisfied. Sen. Ben Nelson, D-Neb., a conservative needed by Mr. Bush if he hopes to have any support across the aisle, said he pressed the president yesterday to provide a clear and specific mission before ordering additional forces to Iraq. "The White House has to make the case for sending in more troops before they send the troops," he said. "We need a new direction, not just a new slogan."

And even many Republicans appear unenthusiastic about troop increases as well. Senate Minority Whip Trent Lott, R-Miss., said Thursday night on MSNBC's "Hardball" that he might say no to the surge. "I want to know what it all is," Mr. Lott said of Mr. Bush's overall plan. "But here's my main point: We've got to change the status quo. At some point, we've got to say to the Iraqis, 'Congratulations. Saddam is dead. We've given you an opportunity for peace and freedom. It's yours.' "

The letter by Ms. Pelosi and Mr. Reid sent a signal that the new congressional leadership intends to be aggressive in voicing opposition to Mr. Bush's handling of the war. With their new majorities, they have a bigger political megaphone and more ability to bring pressure to bear. Yet at the same time, the Democratic leadership has eschewed using the main legislative mechanism to change policy, cutting off funding for the war.

"Surging forces is a strategy that you have already tried, and that has already failed," Ms. Pelosi and Mr. Reid wrote. "Like many current and former military leaders, we believe that trying again would be a serious mistake. ... Adding more combat troops will only endanger more Americans and stretch our military to the breaking point for no strategic gain."

By releasing the sternly worded letter, Democratic leaders hoped to jump ahead of Mr. Bush and set the agenda for the weekend talk shows. Rep. Rahm Emanuel, D-Ill., chairman of the House Democratic Caucus, said the party wants even to address the terminology of the White House plan -- defining it not as a surge, but as an escalation.

While the din of opposition rises, the administration has yet to make a public case for why more troops would be the answer. Even senior military officers have expressed deep skepticism in public and outright opposition in private. Virtually the only prominent voices advocating the surge option are Sens. John McCain, R-Ariz., and Joseph I. Lieberman, I-Conn., and some neoconservative scholars.

Mr. Bush installed new U.S. figures yesterday to manage efforts in Iraq. As previously reported, he replaced Gen. John P. Abizaid, head of U.S. Central Command overseeing Middle East operations, with Adm. William Fallon, and Gen. George W. Casey Jr., the Iraq commander, with Lt. Gen. David Petraeus, who would be given a fourth star.

Mr. Bush also confirmed that John D. Negroponte, director of national intelligence, will become deputy secretary of state, and be replaced by Vice Adm. Mike McConnell.