Bill Hobbs says the war is not lost, but that the Democrats are determined to lose it:

The war has not been lost. American forces on the ground are in the process of winning it. The American military has never lost a war that the American people and its politicians have vowed to win.
It’s a shame that the Democratic-controlled Congress can’t manage to do anything about illegal immigration, earmarks, sleazy congressional ethics or energy, but it would be better if they buckled down and worked harder at those issues than focusing so intently on legislating American defeat in Iraq.
Do they really want to run in ’08 as the party that caused us to lose in Iraq?

(Via Glenn Reynolds.)
I agree with Bill Hobbs, and I think the strategy is to do everything possible to lose the war in such a way that the Republicans will be blamed. One of the ways this is being done is by discounting the documented military brilliance of experts like General Petraeus, whom the Democrats themselves voted to appoint!
Obviously, this is a tricky business, requiring cunning, duplicity, and deception.
Comedian Jackie Mason (while a bit blunt) shows that he understands the dynamics of this elastic inconsistency quite well in this YouTube video:

I have not fact-checked (and cannot vouch for) everything Mason says, but what I find delightful about the video is that he manages to nail the Democrats on their strategy by applying simple common sense. (No wonder the latter is under attack everywhere.)
Mason is absolutely right about one thing: the Democrats did vote unanimously to confirm General Petraeus. (And most of them supported entering Iraq.)