Bursts of gunfire and bad news are prompting growing numbers of foreign policy experts to begin debating the contours of a U.S. military withdrawal from Iraq, echoing a national discussion reflected in opinion polls about the fate of the American mission.

Few prominent American voices have demanded the immediate exit of the 138,000 U.S. troops now leading the Iraq occupation, but a number of commentators have argued in recent days for establishing a date or a concrete list of tasks whose completion would trigger a U.S. departure.

"The destruction of the Baathist regime is the fullest expression of liberation that we can accomplish," said Boston University professor Andrew Bacevich, a retired Army colonel who favors a pullout. "It is simply beyond our ability to bring into existence a liberal democratic order, and to persist in attempting to do so is, first of all, to end in failure."

Win Without War, a coalition of 42 antiwar groups, called Thursday for the setting of a withdrawal date on the grounds that the presence of an "unwelcome occupation force" and the ill treatment of Iraqi prisoners are strengthening the bloody anti-American insurgency.

Even so, neither Bacevich nor Win Without War is demanding an immediate pullout. As with others who are uneasy about the U.S. role in Iraq, the antiwar organization is torn, with some members favoring a quick exit amid mounting casualties and others believing troops should leave only when Iraqis have a government of their choosing.

"People feel strongly that the United States has an obligation to the Iraqi people, given what we are responsible for," said Tom Andrews, national director of Win Without War. He maintains that U.S. public opinion reflected in the polls is moving faster than Washington-based politicians, policymakers or pundits.

A Washington Post-ABC News poll last week found that 40 percent of respondents thought U.S. troops should be withdrawn to avoid further casualties, even if it meant civil order would not be restored. A Gallup poll in early May found that 29 percent wanted to withdraw all troops, while an additional 18 percent wanted to withdraw some. Twenty-five percent wanted to send more.

Polls of Iraqi public opinion show a majority in favor of an immediate U.S. withdrawal.

Among national security specialists who favor setting a date, none suggests a U.S. withdrawal before expected January elections for a constituent assembly. Many say American forces should remain through 2005 to provide security for a constitutional referendum and national elections.

Retired Marine Gen. Anthony C. Zinni, a sharp critic of the Bush administration's handling of the Iraq conflict, said it is too early to choose a date.

"It would give the bad guys and those who want us to fail a target. And when you start to think exit strategy, you start doing things to minimize your cost," said Zinni, who favors making an assessment after the June 30 political handoff to an interim Iraqi government.

President Bush has often said U.S. forces will remain in Iraq as long as necessary, and he has not set a timetable or explicit preconditions for a pullout. Before the war, the Defense Department hoped for a far more rapid reduction of U.S. strength in Iraq. But a stubborn insurgency that has killed 476 Americans and wounded more than 4,100 forced a change of plans.

In his May 24 speech to the U.S. Army War College, Bush said it is imperative that the United States prevail in Iraq and bequeath a representative democracy. Calling Iraq "the central front on the war on terror," he said a "return to tyranny" would embolden terrorists to strike around the world.

"We may have to leave earlier than we thought," said former U.S. diplomat Morton Abramowitz. "Setting a date is probably, under the circumstances, the best way of concentrating the mind. The only ones who are going to save us are the Iraqis. The shorter we can cut the occupation, the better."

Abramowitz believes it would be a mistake to withdraw right away, but he advocates designing a departure strategy and announcing it in the name of transparency. He thinks an appropriate date would be sometime before the end of 2005, a view shared by James Steinberg, former deputy national security adviser in the Clinton administration.

Steinberg argues that the Bush administration should pledge to end the military occupation by the end of next year -- after the adoption of a constitution and national elections. One reason is to dispel Iraqi suspicion that the United States has long-term designs on the country and its oil.

"The more we talk about staying 'as long as it takes,' the more it appears we are trying to impose our vision on Iraq -- further alienating the Iraqi public," Steinberg and Brookings Institution colleague Michael O'Hanlon wrote in a May 18 op-ed article in The Washington Post.

They worry that the Iraqis will insist the Americans leave before the country has become stable, creating a security vacuum "that could ignite civil war and wider regional strife." Likewise, if the United States pulled out now, Steinberg said in an interview, "there's almost no chance you would have a meaningful election because you wouldn't have the security environment."

At the United Nations, where the Bush administration is seeking Security Council approval for the creation of an interim Iraqi government, Algeria's U.N. ambassador has said the United States should include a firm date for the withdrawal of foreign troops.

"I believe everybody would like to see a date for the end of the presence of foreign forces," Abdallah Baali said. "Now we have to find a way to do it."

Most countries -- including Iraq war opponents France, Germany and Russia -- have taken a more nuanced approach. They say the formal mandate for foreign forces should end when a new Iraqi government is elected. At that point, Iraqi authorities could decide whether to ask outside troops to stay.

China, meanwhile, circulated a paper at the United Nations last week asking that the Security Council be given the authority to decide whether the multinational force can remain in the country after elections.

A key reason not to withdraw before Iraq has stabilized, said retired Army Gen. George Joulwan, former supreme allied commander of NATO, is the danger of deadly attacks on huge departing convoys. The image he suggests is of American forces fighting their way out of the country, just as they fought their way in.

"If you're under pressure, if you're under fire, you have to set up perimeters to protect your pullout area," Joulwan said. "Remember how Saigon looked when we pulled out."

Staff writer Colum Lynch at the United Nations contributed to this report.