Looking into my crystal ball, I predict that if Obama gets the nomination, we will start to see his position become more nuanced, and that "16 months" or "18 months" or whatever it is this week will become a "goal" to extract a "large number" of soldiers, not the entire group.
I also predict that he will "discover" that a plan for withdrawal is not quite as easy as he makes it out to be. He will need extra time to "coordinate" the withdrawal with our Iraqi "partners."
Furthermore, any "delays" will be strictly due to something or other W did (Status of Forces? Budget? Lack of planning?), not anything forseeable by candidate Obama.