The excuse has to be something nobody can appear to be supporting (pedophilia, terrorism, nazis, etc.). It's not only an appeal to emotion, it's also a false dichotomy, a loaded question, guilt by association.

Others look at this recipe and can't help but notice its effectiveness. Eventually nobody is beneath pulling this kind of logic, even if they were the ones crucifying it just a few short years ago. The weaker the leader, the more likely that that they forget where they wrote down those principles of theirs and resort to this crap.