Dukakis picking Lloyd Bentsen as a running mate.
This was arguably a mistake as Bentsen overshadowed Dukakis.
Well, Sanders 2020 was a very well-run campaign. Probably would have won, too, if not for the "electability" panic that the party got into.
Bernie in 2016 would have beat Trump, his favorable and GE numbers were much better than Hillary's. For some reason by 2020, he had lost his mojo, his favorability and GE numbers were mediocre. Some say him changing his emphasis from anti establishment in 2016 to #SOCIALISM in 2020 was the cause.
I would say that it was his messaging in 2016 that was much better than it was in 2020. His positions on the issues were hardly different in 2020 than they were in 2016 and healthcare and economic inequality were just as front and center in 20 as they were in '16, the only thing that was different in 2020 was that he went in a more "woke" direction.
