This is a great article that I think sums up the Democrats strategy to sabotage victory in Iraq to secure a favorable 2008 election.

If the war in Iraq is such an unnecessary and futile expenditure of blood and treasure as Pelosi et al. have been saying, why not put an end to it?
But to do that would mean taking responsibility for the consequences – and those consequences would be disastrous and lasting. They would probably still be lasting when the 2008 elections come around.
The Democrats cannot risk that. They have taken over Congress by a very clever and very disciplined strategy of constantly criticizing the Republicans, without taking the risk of presenting an alternative for whose results they can be held responsible.

The problem is that the Democrats are being held accountable, by there own base. They cannot have it both ways, criticize but offer no alternative. It is just a scapegoat for not taking any responsibility. Those who voted for them, those who are anti-war, will hold them accountable and they cannot hide from them.
It certainly has painted them into a corner.