NEW : Sen. McCain : withdrawl should only begin when conditions warrant it . Democrats and Obama critics say troops should stay until success is assured . U.S. has said withdrawal would start in January . Appearing on ABC 's `` This Week , '' Sen. John McCain , R-Arizona , questioned the wisdom of a firm deadline to start withdrawing forces . Withdrawal also depends on situation on ground , Obama says .
NEW : Sen. McCain : withdrawl should only begin when conditions warrant it . Democrats and Obama critics say troops should stay until success is assured . U.S. has said withdrawal would start in January . Withdrawal also depends on situation on ground , Obama says .