But Trumpism isn’t just a triumph of marketing or the excrescence of a personality cult. It is a regression to the conservatism of blood and soil, of ethnic polarization and bullying nationalism. Modern conservatives sought to bury this rubbish with a politics that strikes a balance between respect for tradition and faith in the dynamic and culture-shifting possibilities of open markets. When that balance collapses—under a Republican president, no less—it may never again be restored, at least in our lifetimes.

For liberals, all this may seem like so much manna from heaven. Mr. Trump’s nomination not only gives his Democratic opponent the best possible shot at winning the election (with big down-ballot gains, too), but of permanently discrediting the conservative movement as a serious ideological challenger. They should be careful what they wish for. Mr. Trump could yet win, or one of his epigones might in four or eight years. This will lead to its own left-wing counter-reactions, putting America on the road to Weimar.