There was clearly a winning path with say, Bernie in 2016. The state by state Bernie/Trump matchup polling data consistently predicted a clear and decisive victory. Or, maybe Estes Kefauver 1952, or go back to the 40s and Gallup predicted Henry Wallace would have had a 1936 style landslide instead of the squeak they won with Truman.
As a hobby I've poured over archives of primaries, old newspapers, speeches, going back even to Hannibal Hamlin, Lincoln's first VP and how he got replaced.
I continue to claim that any actual left project (as opposed to whatever the propaganda industry is deciding to imagine the left is) would be far more successful under a Republican flag because they aren't as committed to the neoimperialist project.
That's why the Democrats had all the warring Republicans on their side this time.