this post by Ezra gets at another aspect of the way our politics works that seems completely broken. This dyanmic, fundementally, is what leads to the eternal lucy and the football meme that atrios and others point out. To an outside observer like me, what's most surprising isn't that "moderate" compromises become controversial given the dyanmics of modern american politics, but rather that this development seems entirly surprising to democratic leaders. They keep engaging in good faith compromises and discussions hoping to create a "bipartisan" bill, where the current political and instituional incentives mean that republicans will be rewarded for causing democrats to not pass bills and that they have the power to do that very thing.
The frustating part is that I'm pretty sure that when democrats are in the minority, they won't play by these rules, much as they didn't during the Bush years. Of course, given the still existant media bias towards republicans, its probable that if they attempted to play by republicans rules they'd be murdered by the media that has ignored the republicans doing the same thing.