The massacre of at least 16 Afghan civilians, apparently by an American soldier, forced the Obama administration Sunday to confront yet another nightmare from the war zone and fresh evidence that patience back home is increasingly wearing thin.

A majority of Americans — 55 percent — believe that most Afghans are opposed to what the United States is trying to accomplish in that country, according to a new Washington Post-ABC News poll. About as many Americans — 54 percent — want the U.S. military to withdraw even before it can train the Afghan army to be self-sufficient, a pillar of President Obama’s war strategy.

While most Democrats and independents soured on the war a long time ago, the poll found that Republicans, for the first time, are evenly split on whether the ­decade-long war is worth fighting.

The divide was reflected in Republican presidential contender Newt Gingrich’s call Sunday to withdraw. “I think we’re risking the lives of young men and women in a mission that, frankly, may not be doable,” he said on “Fox News Sunday.” Among Republican candidates, that view puts Gingrich closer to the position of Rep. Ron Paul (Tex.), who has long called for an end to the war.

In contrast, former Massachusetts governor Mitt Romney and former senator Rick Santorum (Pa.) have staked out the opposite position, criticizing Obama for pledging to withdraw U.S. combat troops by the end of 2014. They have said that declaring a firm timetable for ending the war is a sign of weakness and puts American troops at risk.

Obama generally receives high marks from voters for his handling of national security, especially since May, when he ordered the daring helicopter raid on Pakistan that resulted in the death of Osama bin Laden. But news from the Afghanistan front has been uniformly poor in recent weeks, raising the question of whether what had been seen as a political strength for the president could turn into a liability as the November election nears.

‘The worst thing’

In Washington, U.S. officials huddled in the White House and the Pentagon to put together plans for dealing with the repercussions of Sunday’s killings. Obama and Defense Secretary Leon E. Panetta placed separate calls to Afghan President Hamid Karzai to condemn the shooting rampage and offer condolences.

Notably, however, neither offered an outright apology, in contrast with the U.S. reaction last month after American soldiers — in an apparent mistake — burned copies of the Koran and other Islamic religious texts, touching off deadly protests across Afghanistan. Republicans have previously accused Obama of being too quick to apologize to Karzai and the Afghan people.

In separate public statements, Obama, Panetta and Marine Gen. John R. Allen, the commander of U.S. and NATO forces in Afghanistan, signaled that the United States was committed to its partnership with Karzai’s government and gave no hint that they were reconsidering their overall war strategy.

Their comments echoed messages they delivered this year in the aftermath of the Koran burnings, the fratricidal killings of U.S. troops by their Afghan partners and the release of an Internet video that showed Marines urinating on Afghan corpses.

Given the gravity of the latest killings and the rapid sequence in which the other incidents have occurred, U.S. officials acknowledged that they didn’t know whether they could count on the same approach to work this time.

A senior U.S. official said the administration is “not freaking out yet . . . but you’d have to be under a rock not to think this is the worst thing that could have happened.”

“It plays to the absolute worst fears and stereotypes” Afghans hold of U.S. involvement, said the official, who spoke on the condition of anonymity. “It’s the type of boogeyman Karzai has always raised, but we’ve never had an incident like this.”

A cloud over negotiations

The shooting occurred just two days after the administration hailed progress on the long-delayed strategic-partnership agreement it is trying to negotiate with Karzai, a key part of U.S. military plans to retain a counterterrorism presence in Afghanistan after the final withdrawal of coalition combat troops at the end of 2014.

After many months of negotiations and demands from Karzai, one of two outstanding issues was settled Friday, an agreement on the transfer of detention facilities to Afghan control. U.S. officials were optimistic that the other — whether U.S. Special Operations troops can continue conducting night raids in a bid to surprise and arrest suspected militants in their homes — would be settled by a NATO summit in May, when the alliance wants to draw up final withdrawal plans.

“This makes the absolute worst case for us and our continuing involvement” in Afghanistan, the administration official said. “It’s just awful.”

Overall, 60 percent of Americans believe the war has not been worth the loss in life and expense, according to the Post-ABC News poll, which was conducted Wednesday through Saturday, before Sunday’s attack in Kandahar province. There has been consistent majority opposition to the war for nearly two years.

Staff writers Matthew DeLong and Karen DeYoung contributed to this report.