NEW : Sen. Lindsey Graham : withdrawl should only begin when conditions warrant it . Comment: Obama has said withdrawal would start in July 2011 . Withdrawal also depends on situation on ground , Obama says .
NEW : Sen. Lindsey Graham : withdrawl should only begin when conditions warrant it . Afghan and U.S. critics say troops should stay until success is assured . Obama has said withdrawal would start in July 2011 . Withdrawal also depends on situation on ground , Obama says .