Asked if the war has been worth fighting, 51 percent said it was not, while 47 percent endorsed the mission, according to the Washington Post-ABC News poll.
In July, a narrow majority backed the war as worthwhile.
Only 24 percent said more US forces should be deployed, while 27 percent said the troop levels should be kept the same and 45 percent said the troop commitment should be reduced. In a January survey, only 29 percent said the number of troops should be cut back.
It was the second poll this month that revealed public support for the war fraying.
In a CNN-Opinion Research Corporation survey released earlier this month, 54 percent opposed the US-led fight, with only 41 percent in favor.
Public unease has been reflected in Congress, where some of Obama's fellow Democrats have voiced worries about the open-ended US commitment.
In a speech before veterans on Monday, Obama defended the war as a necessity that was "fundamental" to the defense of the American people in depriving Al-Qaeda of a safe-haven to plot follow on attacks to the strikes of September 11, 2001.
But he warned it would be a difficult fight and that there would no "easy" victory.
Despite signs of public anxiety, a majority of 60 percent approve of how Obama has handled the war, while 33 percent disapproved, the Washington Post poll said.
With millions of Afghan voters due to elect a president for just the second time in the country's history, most Americans had doubts that Thursday's elections would result in an effective Afghan government.
Only 31 percent said they were confident that the vote would produce a government that could rule effectively while a 64 percent majority said they were not confident of such an outcome.
Opinion was divided as to whether the United States was winning the war, with 42 percent saying Washington was winning while 36 percent said the US was losing.
In a potentially worrying sign for the US president, opposition to the war has grown among those who form his core of support -- liberals and Democrats, the survey said.
Nearly two-thirds of staunch Democrats now feel "strongly" that the war was not worth fighting, it said.
And support for the war among voters who identify themselves as liberals has dropped dramatically. Strong approval for Obama's approach to the war among liberals has fallen 20 points and 63 percent of liberals want troop levels reduced.
Backing for the war is running strong among Republicans, however, with 70 percent saying the war was worth fighting, according to the poll.
The United States has about 62,000 troops in Afghanistan as part of a NATO-led force of more than 100,000.
Obama has approved a buildup that will see about 68,000 troops in place by the end of the year but analysts say his commander will likely push for yet more American troops.
In October, the war will enter its ninth year.
US forces led an invasion in 2001 that toppled the Taliban regime but the militants have waged a deadly insurgency against the Western-backed administration and foreign troops, with violence now at record highs.
The survey, carried out between August 13-17, was based on a sample of 1,001 adults and has a margin of error of three points, according to the Washington Post website.
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