I don't think this sort of generalization holds up. Compare books from lefties like Chomsky, Blum, Parenti, Solomon and others with the emphasis on facts, statistics and lots of footnotes with books from Ann Coulter and Bill Bennett. Listen to right-wing radio. This is not to deny there is some smug moralizing among some segments of the left. But I think it is a significant portion of the US right that tends to be big on it.
Here's what film critic Roger Ebert said recently: "When I write a political column for the Chicago Sun-Times, when liberals disagree with me, they send in long, logical e-mails explaining all my errors. I hardly ever get well-reasoned articles from the right. People just tell me to shut up. That's the message: 'Shut up. Don't write anymore about this. Who do you think you are?'"
Tom