Senator John McCain posed this question on the floor of the Senate today:
To those who believe that the best course is to withdraw, I ask: Can you explain to the American people precisely what you believe to be the consequences of this action? If we follow the timetable included in this bil--to withdraw troops whether or not we are succeeding or failing; regardless of whether the country is secured; irrespective of whether the Iraqis can manage their own affairs alone, or whether the forces of terror and chaos will triumph--if we follow this timetable we risk a catastrophe for American national security interests.
You can read the speech in its entirety here.