It goes back to the notion that "fascism is a populist movement without a left option". Fascism has historically coopted language from the left and is doing it today. Here as well as Europe. The latest is the yellow vest protests in Paris where you see both socialists and fascist competing for the messaging and neither ceding the public space to the other. Trumps no dummy in certain arenas, though it helps some people to think he is. His record is cunning self preservation and he'll steal any credit for something that's popular. He used the language of populism on his campaign trail. Mixed in with racism and xenophobia but a good deal was populist. My guess is he saw how well Sanders was doing started talking about the issues that he saw Sanders succeeding with. Issues that the Democratic establishment failed to take seriously at the time. That may be changing now. The public is there on this issue but, unfortunately, the parties donors aren't. See Joe Manchin for an example.