These figures, strikingly, came from both sides of the pre-existing civil war. Early in the campaign, when it seemed as if Jeb Bush had a chance to coast to the nomination as the standard-bearer of the establishment, it was mostly voices from the professional base — talk-radio voices, Fox News voices and for a time Cruz himself — who worked to build up Trump as a populist alternative, to vouch for him and bring him within their faction’s tent.

Then as it became clear that the most establishment-friendly candidates (Bush, Chris Christie, John Kasich, even the more right-wing Rubio) weren’t going to hack it, it was the establishmentarians and self-conscious moderates who decided that Trump was a man they could do business with, not like that crazy Tea Partier Senator Cruz.

Which is how Trump ended up as the candidate of Sean Hannity and John Boehner, Ann Coulter and Jon Huntsman, with Rush Limbaugh running interference for him with the grass roots, and various lobbyists doing the same on Capitol Hill.

Of course many converts to Trumpism were motivated simply by expediency, ambition, power worship. But many were clearly motivated by grudges and fears instilled by the party’s civil war, and by a sense that even though Trump might represent a grave threat to their vision of Republicanism, it would still be better to serve under his rule for a season than to risk putting their hated intraparty rivals in the catbird seat.

The narcissism of small differences, in other words, led both the professional establishment and the professional base to surrender to a force that they had countless ideological and pragmatic reasons to oppose.

For those of us who have long been frustrated precisely by the smallness of those differences, the narrowness of the G.O.P. policy debate, it’s a particularly staggering result: A party whose leading factions often seemed incapable of budging from 1980s-era dogma suddenly caved completely to a candidate who regards much of the conservative vision with indifference bordering on contempt.

So to catalog my wrongness: I overestimated the real commitment of both factions’ leaders to their stated principles and favored policies. (Even though I didn’t agree with many of those policies myself, I assumed from the party’s longstanding resistance to change that someone did!) I overestimated their ability to put those principles ahead of personal resentments. And yes, since to acquiesce to Donald Trump as the Republican nominee is to gamble recklessly with the party’s responsibilities to the republic, I overestimated their basic sense of honor.