Cost: Blame Boehner, McConnell

The Weekly Standard's Jay Cost describes the Trump phenomenon as the manifestation of a "principal-agent" problem between Republican voters (principals) and the party leaders (agents).

Donald Trump is not the Republican party’s real problem; he is a symptom of the problem. There is a generationu-long climate of distrust between conservative voters and Republican politicians. Trump is simply taking advantage of this weakness.

I believe Cost is correct. The whole article is worth your time.