WASHINGTON - American support for the war in Afghanistan has ebbed to new low, with nearly two-thirds of people saying the war no longer worth fighting, according to a new Washington Post-ABC News poll. The finding signals a growing challenge for President Barack Obama as he decides how quickly to pull US forces from the country beginning this summer, the Post said in a front page story. After nearly a decade of conflict, political opposition to the battle breaks sharply along partisan lines, with only 19 per cent 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, the Post said. Only 39 per cent of respondents, however, say they expect him to withdraw large numbers. The Post-ABC News poll results come as General David Petraeus, the US 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 US and coalition forces since last year, Morrell told MSNBC, 'its 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.