There are probably a few Democrats that actually believe in ending the war. I think they're for the most part trying to simeltaneously cater to anti-war sentiments in the public and force compromise on the Republicans. Ultimately something more like the "benchmark" idea will be Democrat policy rather than "phased withdrawals", but what they will do is confirm that the planning thus far has been terrible.
Ultimately even if a truly "cut n' run" Crat gets elected, he/she will be faced with the reality of the situation and the Democrats will essentially have bought the war. You can count on it that the Crats will not leave Iraq so lightly at that point.
The fact of the matter is that, while all the scare tactics about al Queda bases are mostly hype and of little real consequence (the base will be destroyed in the chaos soon after we leave), it is not in any way beneficial for us to leave Iraq to become a failed state (which it would be if we were not doing the "power" job for the Iraqi government)... and to simply pressure Iraq to take control itself is as futile as calling up Musharif in Pakistan and asking "Hey, why haven't you taken control of your country yet?" It's not possible without the power.
Frankly I don't think it matters which party takes office so long as the current administration leaves and the next president promises a more pragmatic approach (less secrecy would be nice too).

If you want a "leave now" president, you'll be sadly disappointed because it is not going to happen.

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"Man lives in the sunlit world of that which he believes to be reality. But unseen by most is an underworld, a place that is just as real... but not as brightly lit... A DARK SIDE!"
-opening from Tales From the Darkside