In teaching logical fallacies as part of persuasion and public speaking courses, I sometimes refer to a half-serious one called "Reductio ad Hitlerum," or "playing the Hitler card."
Debaters who use this fallacy attempt to "prove" that something is undesirable or evil by pointing out that Adolf Hitler or the Third Reich advocated or implemented a similar thing. It can be a fallacy because, of course, Hitler advocated some positive things, like classical music, for example.
Playing the Hitler card tends to derail any argument or conversation, because it--not surprisingly--angers the other side to be compared to a man generally considered to have been evil incarnate. Thus, its use is considered lazy, at best, if not always fallacious.
(There is also the--again half serious--Godwin's Rule of Nazi Analogies that states that all online discussions eventually degenerate to the point where someone compares another's beliefs to Hitler's.)
I was reminded of this as I've observed the recent debacle about Cardinal Francis George's comments comparing some in the GLBT rights movement to the Ku Klux Klan.