Someone who has always been socially liberal but only recently became a leftist. A Sanders supporter, maybe, but Stein 2016/Biden 2020 is the more likely path for that group.
A Democrat who thinks 2020 will be a poisoned chalice.
That's a good point- 2020 may well be a poisoned chalice, but I doubt any voter is thinking like that right now. Quite the opposite, actually. Republicans and Democrats believe that the other party will destroy America if they win, and that they have to be defeated at all costs.