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.
Was the KKK anti-Catholic? Yes, that's a historical fact. Is it possible that some LGBT are what might be described as "anti-Catholic"? Possibly, though some might argue such "anti-Catholicism" is understandable, given the church's position on LGBT rights and the money and energy it spends fighting any expansion of those rights.
Even if Cardinal George's comparison is technically "correct," that doesn't make it appropriate, not to mention PR-savvy. Why he chose it--and continues to defend it--is beyond me.
It seems to me that the Klan is almost as explosive a comparison as Hitler, and thus should be avoided in serious argument, especially those about sensitive issues.
**Another new angle to this story: Some alumni of St. Norbert College in DePere, Wisconsin, who are angered about George's comments think it is inappropriate for him to be the commencement speaker at this year's graduation. See video here .