President Bush made a plea for congressional support for his Iraq War strategies Thursday, but it does not appear likely he will get the kind of resounding endorsement he wants, especially from moderate Republicans in the Senate.

Some of them were highly critical after the speech.

“While I welcome the modest drawdown of our troops that the president has ordered, it is not a sufficient response to the lack of political reform by Iraqi leaders,” said Sen. Susan Collins, R-Maine. “I continue to believe that an immediate change in mission is needed, which would allow a far more significant but responsible reduction in the number of our troops deployed in Iraq. We should not wait another six months to change the mission.”

With crucial votes on the war expected in the Senate next week, Bush is hoping to avoid defections of centrists, who are under pressure to join the Democrats in scaling back the war further and faster than he wants.

Even if Republicans did not desert the president’s war policy in droves this month, as some had expected, several nonetheless split off. Now they threaten to bring others along, as Democrats also search for a way to splinter Republican ranks.

While Bush’s Oval Office speech Thursday was aimed at the nation, highlighting his call to reduce troop levels gradually over the next year, he directed some key passages at Congress.

“Those of us who believe success in Iraq is essential to our security, and those who believe we should begin bringing our troops home, have been at odds. Now, because of the measure of success we are seeing in Iraq, we can begin seeing troops come home,” he said. “The way forward I have described tonight makes it possible, for the first time in years, for people who have been on opposite sides of this difficult debate to come together.”

Beyond predictable statements from party leaders, congressional reaction to Bush’s speech was sparse, with most lawmakers already gone for the week because of the Rosh Hashana holiday.

“As we look forward to our next steps in Iraq, Congress is faced with a stark choice: either rally behind the proven, responsible strategy set forth by General [David H.] Petraeus and bring our troops home after victory, or demand an irresponsible, precipitous withdrawal that will force our troops to leave in defeat,” said House Minority Leader John A. Boehner, R-Ohio.

But Sen. Norm Coleman, R-Minn., who has inched further from the White House each week since August, responded to the speech with skepticism.

He called the troop reduction Bush outlined a “positive development in the short-term,” but said, “Americans need to know there is light at the end of the tunnel well beyond that time frame. . . . America’s role in Iraq is not unending.”

Earlier this week, Coleman said he was working with Democrat Mark Pryor of Arkansas on legislation that would require troop reductions beyond next summer. Coleman, facing a tough re-election battle next year, said he would like to see troop levels cut in half within three years.

On Thursday, Bush said U.S. troop levels in Iraq can be reduced somewhat but, until next summer, will remain at least as high as they were before a recent troop buildup.

Directly addressing Congress in his remarks, Bush said: “Let us come together on a policy of strength in the Middle East. I thank you for providing crucial funds and resources for our military. And I ask you to join me in supporting the recommendations General [David H.] Petraeus has made and the troop levels he has asked for.”

Some wavering Republicans appeared receptive to Bush’s message.

At a Sept. 11 hearing, Elizabeth Dole of North Carolina, also up for re-election in 2008 and who previously had gone along with the war effort, urged a change in mission starting next spring, saying she was compelled to “support what some have called action-forcing measures.”

However, her chief of staff, Brian C. Nick, said before Bush’s speech that Dole does not want to force the president to take any action, because she is “a senator, not a general.”

Several other Republicans are gravitating toward proposals that would indirectly reduce the number of U.S. troops in Iraq by shifting their mission away from policing sectarian strife or by limiting the length of deployments. Such proposals, as opposed to timelines for withdrawal, could gain sufficient Republican support to pass, though not enough yet to overcome a veto.

Collins has joined with Democrat Ben Nelson of Nebraska on an amendment that would force an immediate shift in the U.S. mission, from policing Iraqi neighborhoods to targeting terrorists, training Iraqi forces, protecting U.S. personnel and assets and securing Iraqi borders. Collins believes such a change could bring U.S. troop levels down as low as 50,000.

Democrats Carl Levin of Michigan and Jack Reed of Rhode Island are modifying a measure they wrote that would set forth a similar mission change. It would also require a withdrawal of most troops to begin in 120 days, as the president wants, but with a probable end date in April 2008.

To draw GOP support, Levin and Reed may make the end date a goal, not a mandate. Gordon H. Smith of Oregon is whipping his fellow Republicans on the bill.

Levin is “taking the temperature of his caucus and I’m trying to take the temperature of ours,” Smith said. Levin declined to say which Republican votes he thought he could gain.

Meanwhile, several Senate Republicans are backing a measure by Jim Webb, D-Va., that would require that troops be given at least as much time at home as they spend deployed to Iraq or Afghanistan. Webb said his measure, which netted 56 votes in July, including seven Republicans, now has the likely support of GOP Sens. Lisa Murkowski of Alaska and George V. Voinovich of Ohio.

The amendment, which could be considered the week of Sept. 17, is significant because it would limit the number of troops the president could deploy. It would represent a bipartisan repudiation of Bush on the heels of his speech. The House passed a similar measure in August, and Bush threatened to veto it.

Some Senate Democrats believe their GOP colleagues are teetering on the edge of taking legislative action to hasten an end to the U.S. military role in Iraq.

“A lot of Republicans believe the open-endedness has got to end, the mission’s got to change and there has got to be reductions beyond the pre-surge level, because that’s just a return to the status quo ante,” Levin said. “A lot of them believe that. They will be looking for ways to express that.”

In the House, too, several Republicans have deserted the White House on Iraq. The most recent came this week when James T. Walsh of New York called for bringing U.S. troops home, saying “the clock is running out” on U.S. support for Iraq. But Walsh was not specific about how he wants to start bringing U.S. troops home.

Kathleen Hunter, Bart Jansen and Adam Graham-Silverman contributed to this story.