The GOP establishment has proven unable to stop Donald Trump's candidacy. I blame campaign finance reforms like John McCain, Common Cause, and so on. Why? A strong political party would be able to act as a gatekeeper to the nomination process, stopping extreme candidates from running under the party banner. But an evil combination of self-interested incumbents and moronic reformers have created a system in which contributions to political parties are sharply limited and party control over candidate financing is sharply limited.
Of course, the switch to primaries hasn't helped. That needs to be fixed too.
But changing campaign finance rules so as to strengthen parties would be a great place to start.