As Jonah Goldberg put to one of his liberal freinds: "Gee, Socialized Medicine, environmental laws, full employment...Aside from mass murder, what do you disagree with the Nazis on"
To kid with them some more, point out that Hitler and Mussolini were both vegetarians, then give them Mussolini's (the guy pretty much invented the word facist, or at least helped define it) background, i.e. former editor for 2 different socialist newspapers, and made his name as a union leader.