As is very apparent sad to say.

"But then if he created his own party it would become..." the unmentionable!

can be told is wrong.

As a career elected official, Sanders has been a member of the US Government for most of his adult life.

Of course, some of that has to do with the liberalization of the Democratic Party over the same time and the limits of a two-party system, but still. If Republicans has been a little more "closed" then conservatives wouldn't have been able to turn the "Party of Lincoln" into such a racist organization.

The right of a political party to establish its own rules and processes is covered under "freedom of association" in the Constitution. If any outsider could come in and dictate changes, the party would lose the character on which it was founded and be twisted into something different. It would be no different than men trying to take over NOW, or Christian conservatives trying to take over a gay rights organization.

There is nothing stopping anyone from forming a political party and running it however they see fit. But it involves a lot of hard work, time, and there's no guarantee of success. But by trying to co-opt the Democratic Party, they instantly have a chance at stealing an existing brand and subverting it to their aims, regardless of how other Democrats might feel about that. Fortunately this time the gates held.

"Establishment" to such people is "Anyone who is not us." The way the Sanders campaign maligned every progressive group that didn't endorse him is evidence enough of that at work in this primary. People who can't be good allies will soon find themselves without any, and until these neo-hippie agitators learn how to build coalitions better than they do now, they're going to be outside and angry for many years to come.

Bernie lost my support right early when he called planned parenthood the establishment. It was clear very early in the race that the 'establishment' was nothing more then a dirty word he uses against anyone or anything that doesn't support him.
I keep from bashing him publically in order to be civil, but I honestly think Bernie is scum. Anyone on the left who attacks planned parenthood is scum, and unredeemable in my eyes.

Establishment, The Powers That Be, Them, Flying Money Squid of Death, The Gummint, Banking-Military-Financial-Industrial Complex, Washington Uhleets....

I'm sure I missed some.