I have a theory that non-progressive Democratic voters are so concerned with candidates' perceived "electability" that they're repeatedly picking unexciting candidates who end up being less electable. The Democratic party needs to focus on increasing turnout (a potentially huge pool of new voters) rather than swaying an extra 2-4% of consistent moderate voters over, IMO.

Republican voters don't seem to have the same problem. I mean, Trump was probably the least "electable" candidate of all time. But he got a lot of people excited about voting.