Godwin’s Law, as posed in 1990, was this: “As a Usenet discussion grows longer, the probability of a comparison involving Nazis or Hitler approaches one.”  As internet forums replaced the Usenet, people generalized Godwin’s Law: Given enough discussion about anything, eventually somebody makes an analogy with Nazis and kills the conversation.
It was initially posed as a descriptive law, like gravitation or electromagnetism. In recent years, I’ve seen people more often describe the invocation of Nazis as a “Godwin’s Law violation”. That requires treating it instead as a proscriptive law, a commandment like Thou shalt not make an analogy between your interlocutor and Hitler.
In 2017, we are living in a post Godwin’s Law world. It isn’t funny or provacative to draw analogies with Hitler, nor is it a norm violation to say that something is what Nazis would say, when the current political scene involves actual Nazis.
Of course, President Trump isn’t literally Hitler. But some small fraction of his political base are actual Nazis, he has emboldened them to sieg heil in public, and part of his political strategy is not to distance himself too far from them. When we can ask whether it’s right to punch Nazis, not as a moral thought experiment but as a question about something we saw on the news, then Godwin’s Law just misses the point.