Because (out of his own ignorance) he thinks that he was calling the Democrats nazis. And everyone calls everyone Nazis nowadays.
Now in reality he was misreading Hitler, who attributed the Big Lie to the Jews, not to Nazi efforts. It was a stupid thing to do regardless but I can't my outrage level up that high because damn near everyone I ever hear quoting that "Big Lie" bullshyt thinks that Hitler was talking about Nazi propaganda and doesn't realize he was actually talking about Jewish propaganda.
America even dumber than most people thought and inbred conservative politicians can get away with that.