The conservative columnist for The Wall Street Journal had this to say about calling your political opponent a Nazi (link):

…we disapprove, on both rhetorical and moral grounds, of comparisons between Obama and Hitler or ObamaCare and Nazism. (It should go without saying that such expression is fully protected by the First Amendment.) One should never in earnest liken a political opponent to the Nazis if that opponent does not practice or advocate genocide or totalitarianism.

To do so is a rhetorical error because it calls attention away from the speaker’s message and toward his lack of perspective. It is a moral error because of that lack of perspective. There may be plausible arguments that ObamaCare is evil in intent or would be evil in effect, but it is insane to equate it to the singular evil of Nazism. The easy recourse to Nazi analogies--far more common on the left than the right--debases the currency of moral outrage and can only diminish moral clarity.