Gillum ditched some of his populist progressive rhetoric after winning the primary, and that turned off some progressive base voters. His decision to have people like Hillary Clinton and Debbie Wasserman-Schultz (two relatively unpopular figures) campaign alongside him also didn't help matters. Without these two factors, Democratic base turnout would almost certainly have been higher, and as I mentioned elsewhere, I think base turnout was one of the decisive factors in this election.
IMO, Graham would probably have performed at levels closer to Patrick Murphy in 2016.
Hillary Clinton should not be allowed anywhere on the campaign trail. She is the one of things that unites the GOP. I think it be wise for the dems to distance themselves from her.
