A majority of Republicans say for the first time that the war in Afghanistan has not been worth fighting, according to a Washington Post-ABC News poll that comes as the continuing U.S. presence in that country is emerging as a key point of contention in the presidential race.

The poll findings are likely to present a challenge for Republican front-runner Mitt Romney, who has said that the goal in Afghanistan should be to defeat the Taliban on the battlefield.

President Obama stepped back from that goal during his 2009 strategy review and has set the end of 2014 as the departure date for all U.S. combat forces.

Overall, the Post-ABC News poll reflects a country bone-weary of war after more than a decade of fighting in Afghanistan and, until late last year, an almost nine-year engagement in Iraq.

Public support for the U.S. war effort in Afghanistan has fallen to an all-time low, with only 30 percent of respondents saying it has been worth fighting.

Since the 2001 invasion, almost 2,000 U.S. troops have been killed and more than 15,000 have been wounded in Afghanistan. According to the poll, two-thirds of Americans think the war has not been worth fighting, equaling the most negative public assessments of the U.S. war effort in Iraq.

Although foreign policy has been a peripheral issue in the presidential campaign, the poll’s findings highlight the difficulty Obama and Romney face in explaining U.S. policy to an increasingly war-weary electorate.

Obama, who announced the deployment of 30,000 additional troops to Afghanistan at the end of 2009, is now drawing down those forces with the goal of turning over security responsibilities to Afghan troops by the end of next year. The president intends to bring home all U.S. combat troops by the end of 2014 — and he is tapping into the nation’s war fatigue on the campaign trail.

“For the first time in nine years, there are no Americans fighting in Iraq,” Obama told an audience in Hollywood, Fla., at a campaign fundraising event Tuesday. “And we’ve begun to transition in Afghanistan to put Afghans in the lead, bring our troops home.”

But Romney, whose résuméis thin on foreign-policy experience, has criticized Obama’s management of the Afghanistan war.

In particular, the former Massachusetts governor has said that he would have listened more closely to his commanding generals, who have urged Obama to keep troops in place longer, and not set a specific timeline for withdrawal. Romney says that Obama’s doing so has allowed the Taliban to simply wait out the U.S. military.

“The governor’s position is that Americans are understandably tired of this war, and he recognizes that, in the end, the Afghan people have to prevail,” Richard S. Williamson, a senior foreign policy adviser to the Romney campaign, said in a recent interview. “But we have handicapped them by setting early withdrawal dates, by not being able to forge a more stable NATO coalition and by not broadening the political base in the country.”

The war effort has been rocky, and Obama at times has had trouble convincing his political advisers that Afghan President Hamid Karzai is a trustworthy partner in a risky endeavor to change the course of the war.

As public support for the war has faded — except for the ephemeral bump that followed the killing of Osama bin Laden — Obama heads toward November with Afghanistan as an uncertain asset in his reelection bid.

The Post-ABC News poll found that 48 percent of the public support his handling of the war and that 43 percent disapprove.

Almost two weeks after he outlined his surge policy at the U.S. Military Academy at West Point in December 2009, a Post-ABC News poll found that a slight majority approved of Obama’s management of the war.

For Romney’s campaign, the slip in Republican support for the war could pose political difficulties, placing him outside the majority view of his party. For the first time, more Republicans and GOP-leaning independents oppose the war than support it, with 55 percent saying it has not been worth the costs.

The findings come a month after a U.S. soldier is alleged to have killed 17 Afghan civilians in what witnesses said was a house-to-house rampage. The soldier, Staff Sgt. Robert Bales, had deployed three times to Iraq before arriving in Afghanistan in December.

The poll found that Americans are unsure about what, if anything, the incident reveals about the toll of the war on U.S. troops.

But eight in 10 of those polled say there should be limits on how long service members can be deployed to combat areas.