I’m an aged and skeptical liberal, but I think your characterization of Sanders is accurate: “ His worldview is too one dimensional, his vision of politics too unrealistic. His approach to national security is misguided and his views on trade are fundamentally wrong. He has an obnoxious cult of personality and spokespeople who traffic in alternative facts.”
Still, he’s my second choice after Warren, and I also agree with you that he’s electable (at this point — late February — he seems more electable than Warren).
I think we can speculate profitably about what makes a particular candidate electable, but I don’t think anyone has a clue about what makes a candidate unelectable. At the beginning of the 2016 primaries, I made the outrageous claim on Facebook that Trump would be elected president (based on my reading of his early campaign rhetoric and strategies). My liberal FB friends convinced me that I was wrong, that Trump wouldn’t get the Republican nomination, and even if he did, the general populace wouldn’t go for him (which in terms of popular vote they didn’t). For what it’s worth, I also predicted (even before he threw his hat in the ring) that radio columnist / former Governor Ronald Reagan would be elected president, based on the rhetorical positioning of his radio editorials.
Why is unelectability so hard to measure? I believe the reason is that most people do not vote on the basis of rational self-interest (which can be determined) but the irrational (and thus unpredictable) pull of affiliation. They’ll choose someone from their tribe, regardless of qualifications, even if that person will do less for them than someone from another tribe. Works with sports teams, favorite actors/actresses (Gary Oldman vs. Tom Hanks, e.g.), restaurants — any time people make a choice. It’s one reason why voters distrust congress, while they like the representatives / senators from their state (maybe less so now that the whole country has become so fractured).
Not only did Uncle Ronnie (a mediocre actor) resonate with voters, he pulled off the October Surprise, which helped convince voters that Jimmy Carter was not on their side.
Sanders has persuaded his followers that they’re all on the same team, but can he do this with soft supporters? (Committed Republicans / Trumpers are probably out of the question.)
In my (limited) experience as an aged cynical liberal, the best thing Sanders supporters can do is show that they’re on the same team as the people they’re canvassing. A rational appeal to Bernie’s platform won’t work. The passionately rational voters are already on the Sanders or Warren team, and as you point out, Democrats will probably vote for Sanders even if he’s their last choice on a personal basis. Sanders himself isn’t going to win new converts with his personal style. This is where “It isn’t me, it’s us” becomes truly critical.
