But Trump isn’t just a random demagogue promoting bigotry in some haphazard way. He has an agenda and a message, and it’s a dagger aimed directly at Ryan’s vision for the party. On issue after issue, from trade to immigration to entitlement reform, a Trumpized party would simply bury Ryanism/Kempism under white identity politics, and swing as far from Kemp’s enthusiastic minority outreach as the G.O.P. could get.

One reasonable response to this kind of stark challenge, this incipient revolution, would be soul-searching and a course correction. Trump would not have gotten this far, would not have won so many votes — especially working class votes — if the Kempian vision had delivered fully on its promises, if mass immigration, free trade, deregulation and upper-bracket tax cuts had really been the prescription for all economic ills.

Another reasonable response would be clear defiance, in the style of the “never Trump” movement, based on a recognition that in this election conservatism as we’ve known it could be fighting for its very life, and that if Trump is not repudiated then the American right could be remade in his authoritarian image…

Yet when he’s asked about the threat that Donald Trump obviously poses to “who we are,” the speaker — despite his admirable willingness to condemn specific Trumpian outrages — can’t bring himself to make a counterendorsement, or voice explicit opposition to Trump’s progress.