Not only are Democrats afraid of taking certain kinds of political risks to end the war, but they see no prospect of a political upside to ending it. There was a fairly overwhelming belief in Washington in mid-to-late November 2006 that Republicans would start moving to end the war in January. It didn't happen, but then came the belief that they would start to abandon ship in September 2007, which also didn't happen. But given that Republicans aren't doing what everyone expected them to do and reducing their political exposure on Iraq by winding the war down, Democrats are disinclined to go out on a limb to do it for them.
Drum sees the logic of cowardice:
Upsides first. From the antiwar left … a few congressional leaders who led the fight would get big props, but the rest of the Democratic caucus would get bupkis. Intead, we lefties would probably spend most of our time complaining that they were too late, that they only acted under pressure, that they didn't pull out enough troops, etc. etc. The odds of a genuine political lift are pretty small.
…Now for the downsides.
War supporters, of course, would go ballistic and start blaming every bad event on the planet on the Defeatocrats who pulled the plug on Iraq and betrayed our men and women in uniform. Squishy centrists, most of whom say we ought to withdraw, would probably be apprehensive about voting for someone who actually went ahead did it. Iraq itself would probably get worse if we pulled out, at least in the short term, and there's an outside chance that it would get way worse. Dems would get all the blame, of course. And finally, Democrats would no longer have the war as an issue to run on in 2008.
But there's more. Not only are there fewer upsides than downsides, but the upsides are vague and fuzzy while the downsides are sharp and terrifying and potentially career-ending. This is the underlying dynamic that will probably keep us in Iraq essentially forever, no matter who we elect president. It's all very discouraging.