Once upon a time there was a POTUS candidate who was feared by the party establishment, who was seen as having outlandish ideas about economics & foreign affairs, & who had endorsed extreme ideas that appeared in fringe publications. That candidate was Ronald Reagan in 1980.
Reagan had much support amongst the diehard grassroots of the party, the sort of ideologically committed true believers who followed politics closely. 
But the old guard of the party was suspicious. He seemed too untested, his ideas were not really worth being taken seriously.
Reagan was insufficiently respectful of the way politics had been conducted traditionally, and his economic plans just didn't seem to pencil out.
These screenshots are from this fascinating article by Frank Rich, written in June of 2016. nymag.com/intelligencer/…
Historical parallels are always imperfect and can be as misleading as they can be informative. My only point is that when parties go through major transformations, like the GOP did under Reagan, it comes with much internal strife and stress.
We tend to look back at Saint Reagan and think he was always beloved within the GOP. That's hardly the case. He was an unreasonable, Goldwaterite radical in the 1960s, and was still perceived that way by many in 1980.
You don't have to admire what Reagan accomplished to acknowledge that he profoundly transformed the paradigm of American politics by shifting his party to the right.
A candidate that would shift the Dems to the left comes with stress and strain...but it might also have the same sort of transformative effect on American political culture that Reagan did...only in a different direction.
