The difference is, I think, that Republicans have usually historically chastised Democrats as being morally compromised or lazy and looking for a handout. That's not admirable, but I honestly believe it comes across as less nasty as what Democrats have historically done, and that's chastise Republicans as not *getting it* ("it" being the liberal point of view of the time), aka being fundamentally dumber than Democrats.
Oh, I'm going to have to disagree with that. Maybe things are different outside of Indiana, but the Republicans politicians around here don't say that Democrats are "morally compromised," they say we're godless sinners who will burn in Hell. In the predominantly white, predominantly wealthy area where I live, Republican candidates for county office routinely invoke the threat posed by our (mostly poor, black) neighbors in the next county ('No mass transit! It's not safe to ride the bus with
those people!'). There is definitely an attitude among some conservatives, and wealthy evangelicals are perhaps most prone to it, that liberals are worthless moochers who rank somewhere between cockroaches and child molesters. Sometimes they say we
are child molesters.
I won't deny that Democrats say some nasty things about Republican voters, but I think the question of who is the worst offender depends on where you live and who you're listening to.