Mr. Trump’s most enthusiastic supporters recognized in him, three years ago or more, a permanent occasion for fighting Democrats and the progressive left. He is not an ideological president in the sense of having policy formulas ready to implement; he’s instead a catalyst of conflict with the right’s opponents.

He’s a clarifier. And in some cases, he has clarified questions that don’t resolve into the politics of left and right: when he has shown, for example, how little public commitment there is to bipartisan designs for multilateral trade deals and the exercise of American global leadership through new military interventions in troubled places.

Mr. Trump has done his part to widen the range of what can be discussed at the highest levels of politics. This has been a necessary corrective to the tendency of the country’s political class to close ranks and merge minds.

Mr. Trump has been the nemesis to that class’s intellectual hubris as well. The economy has flourished for three years under a president who pays no heed to economists.