Bernie and Trump are essentially campaigning against the same thing, the entrenched Washington establishment, and the message obviously resonates with the voters. If you trend right then Trump is your guy, you go left, it is Bernie. And the established government is just shitting their pants. Because it looks like it's going to be one or the other who is the next president, and neither will follow the expectations of the bureaucrats.
Trump was a message that the populace hates Washington so they threw a grenade in there. Bernie Sanders is the left's version of that grenade. What the political parties should take-away from this is that the populace fucking hates all of them with a passion. They are less popular than hemorrhoids, Nickleback and herpes. But their jobs depend on not fixing that so we are left with this mess.
But Bernie is the left's version of a disruptive grenade. In a country that's predominantly center right by global standards and by Bernie's standards. In a country in which the performance of the stock market and the robustness of the job market are valued above all things, both of which are currently doing OK under Republican policies. And in America, uniquely in the industrialized world, the job market is inextricably interlinked with access to health care, which Bernie has promised to totally gut.
There's capitalist societies which have a strong social safety net (e.g. the touted "Scandinavian socialists", or, I suppose "Democratic Socialists") and the socialist societies that take over the means of production (the "Socialists Socialists", or, as Republicans say "Commies"). It's not difficult at all to get behind the former, but unfortunately Bernie has a long record of getting behind both. While he may not be singing the latter's praises lately, I have not heard any strong repudiation of his past remarks and associations. He generally is dismissive, with comments like "Why don't you bring up my 3rd grade book reports?" This in spite of the fact that he was grown man at the time. Yes, I do think that will be a problem for him if he wins the nomination, because Trump and the Republicans are going to bash Sanders and the electorate over the head with it.
The main line of attack will be "Bernie is a socialist who will destroy the economy and take away your private health insurance". It will be highly effective. A secondary line of attack will be that "Bernie hates America and has palled around with left-wing communist dicators for decades" It will be moderately effective with older voters. I hope he can overcome this with crossover messaging. So far I haven't seen it. We''ll see if his messaging changes if he wins the nomination. I will be behind Bernie if he wins