Rob makes a salient point about why Democrats can't beat the President on Iraq:
I find it rather telling that the media constantly casts this divide among Democrats as nothing more than simple frustration at being unable to defeat the Bush administration policy-wise in any sort of meaningful way. And while that probably has something to do with it, it’s worth noting that a big reason why the Democrats can’t defeat the Bush administration is that there is a strong contingent of centrist Democrats (or, at least, Democrats representing centrist constituents) who don’t want to defeat the Bush administration on certain issues.
They don’t want a precipitous withdrawal from Iraq. They don’t want an end to some of the anti-terror measures the President has initiated. Among these Democrats are Joe Lieberman, and many other lesser-known figures in the Senate and (to a greater extent) in the House.