There are three models for a winning campaign in 2020. If you believe that the election will be decided by swing voters, you should nominate a realo. If you think there aren’t enough swing voters to matter, you should pick the candidate who will best motivate the base- probably, although not certainly, a fundi. If nothing less than the revolution will do, nominate someone who will downplay the culture war by making concessions to white reactionaries.
Sanders should be running the third kind of campaign. In reality, he is just a base mobilizing guy. That could conceivably win him the election, but it won’t bring about the revolution.