...that crosses all conventional political boundaries, even party affiliation.

Sanders has worked up a subset of the liberal wing of the liberal party. He's surviving within the Democratic primary thanks to an enthusiasm gap, and by not being seriously challenged on his positions. That does not mean he could win a general election.

Liberals loved George McGovern. And they honestly thought they were leading a profound and fundamental movement.

https://en.wikipedia.org/wiki/George_McGovern#/media/File:1972nationwidecountymapshadedbyvoteshare.svg