Well, the Electoral College kills most chances of it to happen on a Presidential level, as Dathi says. You would require a voting bloc as monolithic and radical(ly reactionary) as the South to make a difference. A leftist variant probably couldn't happen, as it won't have enough regional traction.
I was thinking of the Vermont Progressives too, though they would become DFLized very quickly. Progressives VS Democrats could also happen, the Republicans of Vermont becoming absorbed by the latter, but that would leave poor Sanders rather alone in the House or Senate, since he probably wouldn't caucus with the Democrats he fights.