I would argue that politics have actually gotten cleaner (in the US). Violence used to be commonplace in political races, and now are virtually unheard of.
I don't know why that is, because I've heard of plenty of harassment and vandalism against conservative supporters (or were they opposed?) of California's Prop 8 as well, but it didn't get much media coverage. I don't believe one can say that either liberals or conservatives are more "enlightened" overall, at least in matters of raw politics.