The older people among us probably remember how, in 1980, a relatively large number of Democrats crossed party lines and voted for one Ronald Wilson Reagan.
Well, Bernie's doing the same thing in reverse.
This Quora reply to an amusing Quora answer encapsulates it perfectly in micro. And the Facebook Republicans for Bernie Sanders page has a respectable number of likes, over 11,000--without any advertising.
#FeelTheBern!