From the very outset, it has appeared that Bernie Sanders might win with the same sort of crossover appeal that Robert Kennedy seemed to have in 1968, before he was assassinated. Kennedy appealed to a broadening group of liberal and progressive voters (though some still resented his entering the race after Eugene McCarthy had demonstrated the vulnerability of Lyndon Johnson). Oddly, Kennedy also appealed to many disaffected George Wallace voters and might have built a coalition who saw him as a genuine speaker of the truth.
Bernie Sanders seems to be exercising much the same appeal, and might in the general election attract the support of some who are now considering Donald Trump or Ted Cruz.
That is not to say that if Hillary Clinton were the general election nominee she would lose, as did Hubert Humphrey in 1968 (to Richard Nixon). But similar forces are in some ways at work. On the other hand, a Sanders nomination might turn out something like that of George McGovern in 1972--resulting in a landslide for Richard Nixon.
"Robert F Kennedy crop" by derivative work:Robert F. Kennedy appearing before Platform Committee, August 19, 1964.jpg: Warren K. Leffler. This is a retouched picture, which means that it has been digitally altered from its original version. Modifications: Cropped and artifacts removed. Modifications made by Elcobbola. - Image: Robert F. Kennedy appearing before Platform Committee, August 19, 1964.jpg. Licensed under Public Domain via Commons - https://commons.wikimedia.org/wiki/File:Robert_F_Kennedy_crop.jpg#/media/File:Robert_F_Kennedy_crop.jpg