Exactly what happened in the UK. Jeremy Corbyn won the leadership of the Labour party back in 2015 on a radical leftist platform (or what passes for radical these days - would have been mainstream a few decades back). He secured 60% of the vote of party members and affiliated supporters... but prior to this he only just got on the ballot in the first place, barely scraping past the minimum threshold of votes from Labour MPs, who were mostly centrist Blairites. Once elected leader, he faced a constant barrage of attacks from his own MPs, a series of carefully orchestrated resignations and even a leadership challenge that was absurd to the point of pantomime, with the challenger desperately trying to win votes by claiming to support all of Corbyn's own policies (in which case why challenge?). Finally, after losing the 2019 election - which was all about Brexit rather than domestic policy, Corbyn resigned.

Really don't want this to happen with Bernie, but the parallels are striking. There is this huge disconnect between the elected representatives and the voters who they supposedly represent. If Bernie gets past the party, and wins, and then goes up against Trump, will his own party support him, or will they prefer Trump to win because it would mean that next time around Bernie is out and they can pick one of their own instead of this impertinent upstart outsider? Genuine question, because that's what happened here in the UK.