Afghan National Army recruits walk back to their formation after completing marksmanship training during Basic Warrior Training at the Kabul Military Training Center in Kabul, Afghanistan. (Handout/GETTY IMAGES)

Nearly two-thirds of Americans now say the war in Afghanistan is no longer worth fighting, the highest proportion yet opposed to the conflict, according to a new Washington Post-ABC News poll.

The finding signals a growing challenge for President Obama as he decides how quickly to pull U.S. forces from the country beginning this summer. After nearly a decade of conflict, political opposition to the battle breaks sharply along partisan lines, with only 19 percent of Democratic respondents and half of Republicans surveyed saying the war continues to be worth fighting.

Nearly three-quarters of Americans say Obama should withdraw a “substantial number” of combat troops from Afghanistan this summer, the deadline he set to begin pulling out some forces. Only 39 percent of respondents, however, say they expect him to withdraw large numbers.

The Post-ABC News poll results come as Gen. David H. Petraeus, the U.S. commander in Afghanistan, prepares to testify before Congress on Tuesday about the course of the war. He is expected to face tough questioning about a conflict that is increasingly unpopular among a broad cross section of Americans.

Petraeus will tell Congress that “things are progressing very well,” Pentagon spokesman Geoff Morrell said Monday. But because of battlefield gains made by U.S. and coalition forces since last year, Morrell told MSNBC, “it’s going to be heavy and intensive in terms of fighting” once the winter cold passes.

The poll began asking only in 2007 whether the Afghan war is worth fighting, but support has almost certainly never been as low as it is in the most recent survey.

The growing opposition pre­sents Obama with a difficult political challenge ahead of his 2012 reelection effort, especially in his pursuit of independent voters.

Since Democrats took a beating in last year’s midterm elections, Obama has appealed to independents with a middle-of-the-road approach to George W. Bush-era tax cuts and budget negotiations with Republican leaders on Capitol Hill. He called a news conference last week to express concern about rising gasoline prices, an economically pressing issue for many independent voters.

But his approach to the Afghan war has not won over the independents or liberal Democrats who propelled his campaign two years ago, and the most recent Post-ABC News poll reinforces the importance of Republicans as the chief constituency supporting his strategy. The results suggest that the war will be an awkward issue for the president as he looks for ways to end it. Nearly 1,500 U.S. troops have died since the fighting began in 2001.

During his 2008 campaign, Obama promised to withdraw American forces from the Iraq war, which he opposed, and devote more resources to the flagging effort in Afghanistan, which he has called an essential front in combating Islamist terrorism targeting the United States.

After a months-long strategy review in the fall of 2009, he announced the deployment of an additional 30,000 U.S. troops to Afghanistan — taking the total to more than 100,000 — and a July 2011 deadline for the start of their withdrawal.

The number of respondents to the Post-ABC News poll who say the war is not worth fighting has risen from 44 percent in late 2009 to 64 percent in the survey conducted last week.

Two-thirds of independents hold that position, according to the poll, and nearly 80 percent said Obama should withdraw a “substantial number” of troops from Afghanistan this summer. Barely more than a quarter of independents say the war is worth its costs, and for the first time a majority feel “strongly” that it is not.

Obama, who met with Pe­traeus on Monday at the White House, has said he will determine the pace of the withdrawal by assessing conditions on the ground.

At the same time, U.S. and NATO forces have come under sharp criticism from the Afghan government. Over the weekend, after a NATO bombing killed nine children, Afghan President Hamid Karzai demanded that international troops “stop their operations in our land,” a more pointed call than previous ones he has made following such deadly NATO mistakes.

The telephone poll was conducted March 10 to 13 among a random national sample of 1,005 adults. Results from the full poll have a margin of sampling error of plus or minus 3.5 percentage points.

The survey also asked respondents to assess Obama’s performance in managing the political changes sweeping across the Middle East and North Africa. Overall, 45 percent of respondents approve of his handling of the situation, and 44 percent disapprove.

In Libya, where Moammar Gaddafi is battling a rebel force seeking to end his 41-year rule, Obama is under increasing pressure to implement a no-fly zone over the country to prevent the Libyan leader from taking back lost territory and to protect civilians from government reprisals.

Nearly six in 10 Americans say they would support U.S. participation in a no-fly zone over Libya, the poll found, despite recent warnings from Defense Secretary Robert M. Gates that doing so would be a “major operation.”

But the survey found that American support dips under 50 percent when it comes to unilateral U.S. action, as Democrats and independents peel away.

When told that such a mission would entail U.S. warplanes bombing Libyan antiaircraft positions and “continuous patrols,” about a quarter of those initially advocating U.S. participation turn into opponents.

After a meeting Monday with Danish Prime Minister Lars Loek­ke Rasmussen, Obama said, “We will be continuing to coordinate closely both through NATO as well as the United Nations and other international fora to look at every single option that’s available to us in bringing about a better outcome for the Libyan people.”

In general, Americans do not think thatthe changes in the Middle East and North Africa will prove beneficial to U.S. economic and security interests.

More than seven in 10 respondents said demonstrators are interested in building new governments, although not necessarily democratic ones. Almost half of those surveyed view the turmoil as undermining the United States’ ability to fight terrorist groups in the region.

Staff writer Karen DeYoung contributed to this report.