The ticket would have lost every state that Biden won by under 5%. Bernie Sanders is an electoral poison. I thank God every day he didn't win the primary.
I've never seen an ounce of evidence for this other than scarred ex-New Left boomers pointing to McGovern in 1972, or thinking that Trump saying "he's a socialist who wants to raise your taxes" would suddenly have shifted all swing voters towards him. In his actual electoral performances in the Senate, Sanders has always
outperformed Generic D, and among politicians with near-universal name recognition he almost always has the highest favorables other than Barack Obama. There were a non-zero number of Romney/Sanders voters in 2012.
I'm not obsessed with relitigating 2016 the way some people are, but I'll never quite get over Democratic primary voters selecting Hillary Clinton on the basis of electability when every poll ever taken showed her performing much worse against Trump/Cruz/Rubio than Sanders, and, of course, when she lost by tanking in the exact same areas where Sanders could have tanked Trump's appeal.