'The Hitler Card' aka 'Guilt by association' aka 'Reductio ad Hitlerum'
Hitler is probably the most universally despised figure in history, so any connection to Hitler, or his beliefs, usually always derails the discussion. Playing the Hitler Card demonizes opponents in debate by associating them with evil, and almost always derails the discussion. People naturally resent being associated with Nazism, and are usually angered. In this way, playing the Hitler Card can be an effective distraction in a debate, causing the opponent to lose track of the argument. However, when people become convinced by guilt by association arguments that their political opponents are not just mistaken, but are as evil as Nazis, reasoned debate can give way to violence. So, playing the Hitler Card is more than just a dirty trick in debate, it is often "fighting words".