“We need to understand that our being in the middle of countries like Afghanistan is probably counterproductive,” Mr. Gingrich said. “We’re not prepared to be ruthless enough to force them to change. And yet we are clearly an alien presence.”

He added on “Fox News Sunday” that he feared the mission was one “that we’re going to discover is not doable.”

Representative Ron Paul of Texas, who has long supported a noninterventionist foreign policy, has long called for a withdrawal of troops from Afghanistan.

The comments by Mr. Gingrich and Mr. Santorum came after news that a United States Army sergeant was suspected of walking off his base in southern Afghanistan and killing 16 Afghan civilians, many of them children.

At a campaign appearance in Mississippi, Mr. Santorum, a former Pennsylvania senator, said the United States should apologize if it is shown that these killings were “a deliberate act.”

In another appearance on Monday, in Tuscaloosa, Ala., Mr. Santorum also said that he is “the only candidate in this race that has any experience as commander in chief.” Aides later said he misspoke.

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Despite their most recent comments, Mr. Santorum and Mr. Gingrich have both criticized the withdrawal timetable laid out by Mr. Obama. So, too, has Mitt Romney, the former Massachusetts governor.

Mr. Romney has said he would rely on advice from military commanders for his Afghanistan policy, adding last summer that it was “time for us to bring our troops home as soon as we possibly can, as soon as our generals think it’s O.K.” He has also said he would not negotiate with the Taliban.

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“He’s definitely given himself wiggle room,” Daniel Drezner, a professor of international politics at Tufts University, said of Mr. Romney’s policy toward the country.

What’s more clear, Mr. Drezner added, is growing public opposition to a conflict some once described as “the good war.”

“There’s no question that there has been a rising tide of, ‘Why are we in this conflict now,’ ” he said. “And so as much as Republicans might want to sound hawkish, it’s tough to sound hawkish on a conflict where your rationale for being there has evaporated.”

“That said,” he added, “remember that these guys are fighting for hard-core G.O.P. primary voters,” some of whom believe the United States should fight until victory.

But even within the Republican Party, the numbers of such voters may be dwindling: Republicans are now evenly split over whether the war in Afghanistan has been “worth fighting,” according to a new poll by ABC News and The Washington Post. That is the least amount of support over the past five years the question has been asked. In February 2007, 85 percent of Republicans responded yes to the same question.

Over all, 60 percent of those surveyed this month said the war has not been worth fighting. Only 28 percent of Democrats — and 33 percent of independents — believe the war has been worth fighting.

The poll was conducted before news of the killings in southern Afghanistan, but after the Koran-burning episode last month that led to huge protests and was blamed for attacks that led to the deaths of several American service members.