I'm not familiar with Clarus Research Group, but they did poll 1,000 people and 68 percent of them said they think Afghanistan is "a conflict that will go on and on without a clear resolution." If you believe the research of political scientist and former Bush White House advisor Peter Feaver, the most important variable in determining whether the public will support an ongoing war is not how it is perceived to be going today, but whether people think victory is possible.
That may help explain why just 38 percent of respondents want to increase troop levels. Forty percent want to "begin to get out," with 14 percent for staying at current levels.
Update: Greg Sargent has flagged two other polls showing public opinion tilting against a troop increase. (The split is 50-41 and 40-37, respectively.) It's hard not to think that episodes like this week's attack in Nuristan province which killed 8 Americans won't hasten that trend.