In politics, the "dog whistle" is coded language designed to delight a targeted subgroup and pass over the heads of everyone else. Other terms, such as "establishment," "Washington insider" and "free trade," are not quite full-grown dog whistles. Let's call them puppy whistles.
These are expressions whose meanings remain vague. For the puppy whistle, the vaguer the better.
Both Donald Trump and Bernie Sanders rail against "the establishment." This is a way of saying that they are not favored by the traditional leaders of their parties -- the leaders said to have let us down.
"Establishment" is hard to define, and when you do, it's sometimes carries positive feelings. Who among us wouldn't be impressed by a plumber's ad reading, "The Wrench Brothers, Established in 1971"?
On the left, "the establishment" is itself a highly established term. It gained steam in the 1960s as a designation for the adults who messed things up for us kids. Sanders uses it as pure pejorative.