"If the people in Afghanistan think we're going to begin to leave in July 2011," Graham said on on CBS' "Face the Nation," "we have no chance of winning a counterinsurgency.
"If you send a signal to your enemies you're going to leave at a certain date, they'll wait until that date and wait you out."
Graham, who is a member of the Senate's Armed Services Committee, said he supports transitioning control to the Afghans. But he said Mr. Obama's deadline strategy has not worked.
"We're not here as an occupier," Graham said. "But this confusion has hurt. It's emboldened our enemies. We need to get it right and get it clarified."
Graham said the purpose of securing Afghanistan is to make America more secure.
"We can't leave this country in chaos," he said. "We'll never be safe. This is not just some place on the map; this is the place where 9/11 was planned and executed.
"If the Taliban ever come back and take over all or part of this country, we'll never know peace," he said.
In addition, Graham said those who'd helped the U.S. would be killed, and jihadists in other areas would be emboldened because "no one will help us in the future."
The senator, who is known for being one of the most willing Republicans to work with Democrats, said he wants to hear the White House say that there will be an evaluation in a year's time.
"If you can transition, we will do so in certain areas. But everything will be conditioned," Graham told moderator and CBS Political Analyst John Dickerson. "The vice president reassured me that it would
be condition-safe. I take him at his word.
"General [David] Petraeus needs this monkey off his back. It's not fair to him and our troops and our civilian counterparts to be operating in Afghanistan with the belief that come July 2011, we're going to begin to withdraw no matter what.
"That can't be the way we lead this. That would be counterproductive."