WASHINGTON — After the recent defection of prominent Republicans on the Iraq war, the big question in Washington is who might be next.

More than a dozen Republican senators who are running for reelection next year head the list of lawmakers to watch. But others, too, have expressed concerns that the GOP has grown increasingly vulnerable on the issue. As the clock ticks toward Election Day, voter pressure is building against any lawmaker still standing with President Bush on the war.

Potential wildcards include members up for re-election who have broken with the president on other issues such as immigration or who face growing anti-war sentiment in their home states. Those include: Senators Chambliss of Georgia, Stevens of Alaska, Roberts of Kansas, Enzi of Wyoming, Inhofe of Oklahoma, and Sessions of Alabama.

Senator Coleman of Minnesota already has expressed grave doubts about the president’s Iraq policies, but he hasn’t signed on yet to legislation calling for a change in strategy.

Support among Republican senators is considered crucial to Mr. Bush’s Iraq policy. Democrats hold a narrow 51–49 majority and routinely fall shy of the 60 votes needed to cut off debate and advance most anti-war legislation.

But new cracks in Mr. Bush’s support base have begun to show. In the past two weeks, three Republicans — Senators Lugar of Indiana, Voinovich of Ohio, and Domenici of New Mexico — have announced they can no longer support Mr. Bush’s Iraq war strategy and have called on the president to start reducing the military’s role there.

Their announcements took many by surprise because most Republicans have said they are willing to hold out until September to see if Mr. Bush’s troop buildup is working.

“I have carefully studied the Iraq situation and believe we cannot continue asking our troops to sacrifice indefinitely while the Iraqi government is not making measurable progress to move its country forward,” Mr. Domenici told reporters from New Mexico this week. Instead, Mr. Domenici embraced a bipartisan bill by a Colorado Democrat, Senator Salazar, that would put American troops on track to leave by the end of March 2008.

A spokesman for the White House, Tony Fratto, said that position amounts to the same approach sought by the Democrats, “which is, in fact, a precipitous withdrawal.”

“We think that’s absolutely the wrong way to go,” Mr. Fratto said Friday. “It would be dangerous.” Mr. Domenici’s remarks were a switch for the 34-year Senate veteran and GOP stalwart. Just three months earlier, he scolded Democrats for a proposal to fund the troops but order them home this fall.

While he is still likely to oppose such legislation, Mr. Domenici’s rhetoric has changed substantially since April when he said he was committed to giving the military the “time and resources to try to calm Baghdad.”

Mr. Domenici’s term in Congress expires next year, alongside 20 other GOP senators. Of those, a dozen or more are expected to run for re-election. Four have signed on to Mr. Salazar’s legislation: Mr. Domenici, Senators Collins of Maine, Sununu of New Hampshire, and Alexander of Tennessee.

Senator Smith of Oregon, also running for re-election, came out earlier this year in support of separate Democratic legislation ordering troops home this fall. The wildcards in the debate are Senators Roberts, Stevens, and Chambliss, who have staunchly defended Mr. Bush but are watching his poll numbers drop.

Others include lawmakers like Senator Bond of Missouri who won’t face voters next year but want to take back control of Congress from the Democrats and have expressed concerns about the lack of progress in Iraq.

Senator Warner, whose term is up in 2008 but who is undecided on whether to run again, is expected to propose legislation this month calling for a new strategy. Sensing the shift, administration officials have reached out to Republicans posing alternative scenarios in Iraq to gauge political support, according to one Senate aide.

Senator Snowe, among the first Republicans to call for a phased withdrawal of troops, said she wouldn’t be surprised to see more of her colleagues follow suit because the Iraqi government has failed to live up to most of its political promises. She is not up for re-election next year.

“I think [Domenici] is reflecting that depth of frustration” among all members, said Ms. Snowe, a Republican of Maine, in a phone interview Friday. “The big question is exactly what everyone’s going to be able to support that represents a change in course.” As pressure builds for a change in Iraq policy, a top American commander there warned Friday that drawing down troops too soon would create more problems.

“You’d find the enemy regaining ground, re-establishing sanctuary, building more” roadside bombs, Major General Rick Lynch told a Pentagon news conference. “The violence would escalate. It’d be a mess.”