At this point, I don't how the Democratic Party establishment can respond to Bernie without committing suicide. There are one hell of a lot of young idealists lining up for Bernie. I doubt very much that the majority of young people (A term that nowadays means people under 35) have much desire for Bernie. There are a core of young people who deeply believe and a lot more who are swept up in the moment. But, however shallow Bernie's support may be, if he doesn't win it will unleash an astonishing bitterness towards the party.
But let's leave that aside and ask how those of us who aren't in the roughly 25% of the population that forms the core of Democrat support should respond. One possibility that I see a lot on the right is to brand Bernie as a socialist and, therefore, not really American. And I can see why; it seems like a no-brainer. Bernie has declared support for socialism in the past and socialism really is the opposite of what America is about. But I'd like to put another option on the table and that is that Bernie isn't really a socialist. Indeed, most Americans who call themselves "socialist" at one time or another aren't really socialists*.
What Bernie is is where he sits and he sits at the intersection of two deeply embedded American traditions: progressivism and toryism. These are outsider movements, mostly because they have suffered serious defeats in the past. Progressivism's big defeat was the 21st Amendment. Toryism's big defeat was the passage of the Suffolk Resolves. Neither ideology has ever fully recovered from those defeats but neither has gone away either. Combined, they make for a potent mix of moralism and elitist rent-seeking that, left unchecked, will sweep us all into servitude.
Or, to put it another way, he represents something undesirable but not something foreign. The bad things that would come with a Bernie victory are not some funny little foreign dictator we can caricature. No, Bernie represents the sort of injuries we inflict on ourselves.
* Others really are and they are cheerfully willing to exploit useful idiots like Bernie and his supporters but that's another issue.