He praised Trump when he thought Trump would flame out and not attract support but not before he undermined the establishment candidates. While he killed the establishment, he also killed off Cruz. That was not the plan.
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I think you have put your finger on something.
The power structure -- politicians, academics, media -- has to tolerate a certain amount of free speech and dissent. Trump was fine, just dissent, as long as he was denied all but his voice. There are numerous examples of characters like this, Ron and Rand Paul, candidates in minor parties (on both ends of the political spectrum). These dissenting voices can be observed, discussed, cajoled and even embraced - as long as they don't take popular hold.
Limbaugh is part of the controlled dissent. A vent on populism, a way for it to blow off steam without effecting any meaningful change.
That way of life is now under pressure. People are waking up to the notion that they have been manipulated and taken advantage of.