By Jeff Mason and Andy Sullivan
WASHINGTON (Reuters) - What a difference a few weeks can make.
Just as President Barack Obama was enjoying the fruits of an improving economy and a bitter fight among Republican presidential contenders, rising gas prices and a massacre in Afghanistan have put a spotlight on potential vulnerabilities for the Democrat's re-election campaign.
The killings of 16 civilians in an Afghan village on Sunday, allegedly by a U.S. Army sergeant, were a reminder that unpredictable world events can suddenly change the narrative of Obama's bid to hold on to the White House in the November 6 election -- and give Republicans an opening for criticism.
Although the conflict in Afghanistan began under his predecessor, Republican President George W. Bush, it is now Obama's war.
The Obama administration's timetable for withdrawing from Afghanistan by the end of 2014 -- long blasted by Republican presidential candidates as being too soon -- now can't come fast enough for most Americans.
As new polls indicated that most Americans want the roughly 90,000 U.S. troops in Afghanistan to return, Obama told a Pittsburgh TV station on Monday that the killings made him "more determined to make sure that we're getting our troops home.
"It's time," the Democratic president said. "It's been a decade and, frankly, now that we've gotten (Osama) bin Laden, now that we've weakened al-Qaeda, we're in a stronger position to transition than we would have been two or three years ago."
The shooting is affecting the Republican race, too.
Until recently, only one of the four Republicans battling for the right to face Obama in November -- Texas congressman Ron Paul, who is running fourth in the race -- had called for a quick withdrawal of U.S. troops from Afghanistan.
The other Republican candidates -- former U.S. House speaker Newt Gingrich, former Pennsylvania Senator Rick Santorum and former Massachusetts Governor Mitt Romney -- have long criticized Obama's plan to withdraw by the end of 2014. They have said the president emboldened the opposition Taliban by setting a withdrawal date.
Now, Gingrich and Santorum are suggesting that it might be time to wrap up the U.S. mission in Afghanistan quickly -- while continuing to cast Obama's handling of the situation as a lack of leadership.
Santorum said on Monday that it might make sense for the United States to withdraw before 2014 because Obama's timetable was giving hope to U.S. enemies.
"Any time you have such a shocking development I think it's important to look and see what the situation is and whether it's possible to continue on," he said on NBC on Monday.
Without a commitment to staying until Afghanistan is more stable, Santorum said, "I don't see any reason why we should continue to hold on to an artificial date, which is obviously not working for us."
On Sunday, Gingrich said Washington should look at pulling out of Afghanistan and reconsidering its role in the region.
"There's something profoundly wrong with the way we're approaching the whole region and I think it's going to get substantially worse, not better," he said on "Fox News Sunday."
"I think that we're risking the lives of young men and women in a mission that may frankly not be doable," he said.
Republican front-runner Romney, a vocal critic of Obama's withdrawal timetable, took a more cautious approach. He said the slayings of Afghan civilians should not alter U.S. policy.
"The actions of a deranged person are not going to shape American foreign policy," he said on Fox Business Network.
"That being said, we should on a regular basis reassess what is happening in Afghanistan, and any place, for that matter, where we have kinetic activity going on, and assess what is the right course forward."
PUBLIC LOSING PATIENCE WITH WAR
The evolving views of the war among the Republican candidates come as the United States has suffered several damaging incidents.
Sunday's massacre followed controversial incidents in which U.S. troops urinated on the corpses of suspected Taliban fighters in January and burned Korans taken from detainees in Afghanistan in February.
A Washington Post poll released on Monday -- and taken before Sunday's killings -- said 60 percent of the U.S. public did not consider the war worth fighting, and 47 percent disapproved of the way Obama had handled the war compared with 46 percent who approved.
"Public opinion about the war has moved steadily ... in the direction that we ought to pull out, and the sooner the better. This massacre will certainly enhance that," said Mark Rom, a politics professor at Georgetown University.
"Will that hurt or help Obama? It's going to hurt any Republican who says we ought to double down on the war. The public just doesn't have the stomach for that."
The sentiments reflected in the poll, combined with growing anger over rising gasoline prices, have put pressure on Obama's campaign just as it was riding a wave of positive economic news.
The jump in fuel prices is driven in part by tension with Iran, another foreign policy issue that could overshadow good news on the economy or put a brake on the economic recovery.
Obama criticized Bush during the 2008 campaign for not paying enough attention to Afghanistan, and Obama has overseen a surge in troops there.
The United States has lost about 1,900 troops during the decade-long conflict.
Washington and Kabul are trying to negotiate an agreement that would allow U.S. advisers and special forces to stay after combat troops leave at end of 2014.
(Editing by David Lindsey and Cynthia Osterman)