There's a bunch of reasons why that's true, but here's the simple test. Compare these two attacks and see who's going to come out ahead:
REPUBLICANS: Democrats demonize people of faith!
DEMOCRATS: Republicans demonize atheists!
Yeah, that's going to turn out well. Or try with different categories. Families versus homosexuals. Soldiers versus dissenters. Immigrants versus "hard working Americans."
Note that I am not saying that these dualism are accurate characterizations of the issues. I'm merely pointing out that when things get reduced to their barest caricatures, Democrats are going to lose. Because we're a party of deep ideas. We're not good at transforming our message into a taunt, because it's about uplifting America, not trying to keep it down.
One could argue that we're too late, the political climate has already moved past the point of civility, and we need to play dirty or lose every time. I might agree with the first half, but I sorely dissent from the second. If American politics today is framed as a slime contest, then Democrats need to focus their efforts on changing that frame. We need more Barack Obamas, not more Michael Moores.