My take on why Senators Hillary Clinton and Barack Obama decided to vote against the war funding bill, despite some prior ambiguity:
It certainly seems apparent that President Bush is hoping to just wear out the clock on Iraq, and be able to pass the problem on to the next President. I suspect people at the GOP might even be expecting the next President to be a Democrat, and plan accordingly, presenting whoever wins with a "poison pill" writ large, or a flaming bag of dog turd on their doorstep, if you prefer that metaphor. Whichever reasoning they might follow, they don't want to pull out a significant number of troops before the next election at the very least, almost as certainly before the next inauguration.
Because whenever we do start pulling out, yes, there will almost certainly be a bloodbath. And it won't look good for whoever's running things at the time. But it's going to happen sooner or later. Why not just put it off for a year or two, and who cares about the extra casualties in the interim? And maybe by then a Democrat can take the blame; they're probably hoping that that might help them recapture the Presidency in 2012 (assuming a Democrat wins in 2008), and Congress in 2010.
So the Senators might be looking at this, and thinking, if they are successful in their respective bids, that they're the ones who will be stuck holding the bag. And even if they don't win, either in the primaries or in the general election, Obama will be up for re-election in 2010, and Clinton in 2012. So they would have good reason for wanting to get it over with sometime before 2009.