For Sanders in 2020, I would tell him to:
-Reach out to black voters more (especially older ones)
-Go after his primary opponents as aggressively as Obama in 2008
-Focus more on the “electability” argument more
[He didn’t have to change his stances on the issues; he just had to frame his arguments in a different manner.]
I think a good way of turning the electability argument on it's head would have been by actually playing up issues that he was genuinely more moderate on (like gun control) then other Democrats and making the case that that would make him uniquely well positioned to peel back rural voters then other Democrats. Instead he picked hills to die on (like felon voting) that were morally correct IMO but politically short-sighted.
