For the rest, here's your standard-issue horse, lance, and suit of armor. The windmills are that-a-way.
You haven't actually read Don Quixote, have you?
As the original is in Spanish, I can't say I've really
"Tilting at windmills" is more of an English idiom, though, which alludes to attacking imaginary enemies. You see, the #BernieOrBust folks are lashing out at Clinton, but Clinton actually represents their interests better than Trump. If they do something out of spite, like vote for Trump, vote for a hopeless third-party candidate, or refuse to vote at all, then they're attacking Clinton (who isn't their enemy) and helping Trump (who represents everything they claim to hate). I can explain the metaphor in more detail, if you like.
Here's more on the subject: https://en.wikipedia.org/wiki/Tilting_at_windmills
One way of interpreting Don Quixote's tilting at windmills could be as an allegory to promote critical, skeptical, or satirical evaluation of either a hero's motives, rationales and actions, or a nation's foreign policies.
In this case, the "hero" would be the #NeverHillary voter who is nevertheless a Democrat.