How does someone like this even get near a mainstream party in the US?
All it takes to be a Democrat is to check a box when you register to vote (same for any other party in the US). There is nothing stopping anyone who wants to from running for the Democratic nomination. Needless to say, he isn’t endorsed by the Democratic Party and doesn’t have any real popular support. As for why he wants to be known as a Democrat–I’m not an expert on the guy but I do know that he is one of the children of the Kennedy dynasty from the '60s; the Democratic Party is part of his brand, I suppose. He is not a serious candidate, he is almost certainly running specifically to raise his profile and increase the number of followers he has as a professional conspiracy grifter (which has been his occupation for decades now).