Mr. Bush and his aides once thought they could wait to begin those discussions until after Sept. 15, when the top field commander and the new American ambassador to Baghdad are scheduled to report on the effectiveness of the troop increase that the president announced in January. But suddenly, some of Mr. Bush’s aides acknowledge, it appears that forces are combining against him just as the Senate prepares this week to begin what promises to be a contentious debate on the war’s future and financing.

Four more Republican senators have recently declared that they can no longer support Mr. Bush’s strategy, including senior lawmakers who until now had expressed their doubts only privately. As a result, some aides are now telling Mr. Bush that if he wants to forestall more defections, it would be wiser to announce plans for a far more narrowly defined mission for American troops that would allow for a staged pullback, a strategy that he rejected in December as a prescription for defeat when it was proposed by the bipartisan Iraq Study Group.

“When you count up the votes that we’ve lost and the votes we’re likely to lose over the next few weeks, it looks pretty grim,” said one senior official, who, like others involved in the discussions, would not speak on the record about internal White House deliberations.

That conclusion was echoed in interviews over the past few days by administration officials in the Pentagon, State Department and White House, as well as by outsiders who have been consulted about what the administration should do next. “Sept. 15 now looks like an end point for the debate, not a starting point,” the official said. “Lots of people are concluding that the president has got to get out ahead of this train.”

In a sign of the concern, Defense Secretary Robert M. Gates canceled plans for a four-nation tour of Latin America this week and will stay home to attend meetings on Iraq, the Pentagon announced yesterday.

Last week, Mr. Bush’s national security adviser, Stephen J. Hadley, called in from a brief vacation to join intense discussions in sessions that included Karl Rove, Mr. Bush’s longtime strategist, and Joshua B. Bolten, the White House chief of staff.

Officials describe the meetings as more of a running discussion than an argument. They say that no one is clinging to a stay-the-course position but that instead aides are trying to game out what might happen if the president becomes more specific about the start and the shape of what the White House is calling a “post-surge redeployment.”

The views of many of the participants in that discussion were unclear, and the officials interviewed could not provide any insight into what Vice President Dick Cheney had been telling President Bush.

They described Mr. Hadley as deeply concerned that the loss of Republicans could accelerate this week, a fear shared by Mr. Rove. But they also said that Mr. Rove had warned that if Mr. Bush went too far in announcing a redeployment, the result could include a further cascade of defections — and the passage of legislation that would force a withdrawal by a specific date, a step Mr. Bush has always said he would oppose.

“Everyone’s particularly worried about what happens when McCain gets back from Iraq,” one official said, a reference to the latest trip to Baghdad by Senator John McCain, who has been a stalwart supporter of the “surge” strategy. Mr. McCain’s travels, and his political troubles in the race for the Republican nomination for president, have fueled speculation that he may declare the Iraqi government incapable of the kind of political accommodations that the crackdown on violence was supposed to permit.

Officials say that Mr. Gates has been quietly pressing for a pullback that could roughly halve the number of combat brigades now patrolling the most violent sections of Baghdad and surrounding provinces by early next year. The remaining combat units would then take up a far more limited mission of training, protecting Iraq’s borders and preventing the use of Iraq as a sanctuary by Al Qaeda in Mesopotamia, a Sunni Arab extremist group that claims to have an affiliation with Osama bin Laden’s network, though the precise relationship is unknown.

President Bush has repeatedly said that he wants as much time as possible for his 30,000-troop increase to show results. And publicly, administration officials insist that the president has no plans for a precipitous withdrawal — but the key word seems to be “precipitous,” and they appear to be recalibrating their message.

“I think it shouldn’t come as any surprise that we here in the administration, and in our conversations with Congress, and in our conversations with generals on the ground and policy makers in Iraq, are thinking about what happens after a surge,” Tony Fratto, the deputy White House press secretary, told reporters on Friday, at a briefing where he was peppered with questions about the defection of Senator Pete V. Domenici of New Mexico.

That defection followed a similar move by Senator Richard G. Lugar, the ranking Republican on the Foreign Relations Committee.

In the meetings last week, officials say, there was frustration that Mr. Bush’s statements were being drowned out by a presidential race that has created a forum for daily critiques of his policies, past and present.

Moreover, the dynamics inside the administration have changed. The hawks who once surrounded Mr. Bush have been replaced by pragmatists like Mr. Gates, who has made it clear that he wants to lower the political temperature of the Iraq debate at home, and has joined with Secretary of State Condoleezza Rice to gradually shift White House strategy.

When it came time to pick a new “Iraq czar,” the choice was a general who had been openly skeptical about the prospects of the troop increase strategy. Mr. Bush will get a chance to make his case later this week, when he presents an interim report, required by law by Sunday, on the status of 18 “benchmarks” of progress.

The calendar may be working in Mr. Bush’s favor. If he can get through the next three weeks without more defections, Congress will recess until September, returning just as the report from Gen. David H. Petraeus and Ambassador Ryan C. Crocker arrives in Washington.

Also, the Republican defectors have not agreed on what different strategy they would prescribe, giving the president some negotiating room. But Senator Lugar said yesterday on CNN that he would support a significant withdrawal that left “residual forces” in Iraq to ensure that “the whole area does not blow up.”

That approach would mean abandoning the current mission of using those forces to patrol Baghdad and try to reimpose order, which was Mr. Bush’s stated goal in January.

Asked whether he could support an amendment proposed by Senator Ken Salazar, Democrat of Colorado, that would put in legislative language the Iraq Study Group’s call for a withdrawal of combat units by March 31, 2008, Mr. Lugar said it was “worthy of a lot of discussion.”

John Hamre, the president of the Center for Strategic and International Studies, who was headed to Baghdad over the weekend to begin preparing another Congressionally mandated report, an independent assessment of the Iraqi military, said, “The political power of Salazar’s amendment is its ambiguity.”

“What does it mean?” Mr. Hamre asked. “That we will immediately implement all 76 provisions? I doubt it. It’s a way to give political cover.”

Senior officials involved in preparing the report Mr. Bush must deliver to Congress this week say he will be able to praise the Iraqi government for delivering the troops it promised — if a little late — and for removing the restrictions on arresting or killing violent members of Shiite militias. But on the critical issue of political compromise, Mr. Bush will be able to report little progress.