This fretful ambivalence turned into enthusiastic commitment over the next four years. Mr. Trump was far better on policy for conservatives than skeptics had imagined. His judicial selections, tax and regulatory moves, and toughness toward China and Iran earned their favor. He and the conservatives also shared enemies — a biased media, an entrenched bureaucracy, liberal Democrats. And because many of the attacks on the president seemed to conservatives to be partisan, silly or false, it became easy to pass over those criticisms that were objective, serious and true.

Many Republican officials who had voiced concerns over Mr. Trump’s style and agenda retired or died. The NeverTrump faction of Republican and conservative elites broke off from the movement to found alternative media platforms and institutions. Most of the central institutions of the American right — the activist think tanks, the single-issue groups, talk radio, blogs, cable news — aligned themselves with Mr. Trump’s program, if not always with his persona.

It was a tricky play. The Trump program is subject to change at the whim of the Trump persona. The party more often catered to the president’s obsessions, tastes, moods and inclinations than it stood against them. Even as it became clear that what really thrilled the crowds at MAGA rallies was Mr. Trump’s unpredictability, brashness, crudity, dark comedy and unapologetic fighting spirit, some on the right began an effort to backfill ideological content into the vessel of “Trumpism.”

What they forgot was that for Mr. Trump, everything is a transaction. Deals can be modified until the last moment and then litigated after the contract has been signed. It’s true that the broad outlines of Mr. Trump’s worldview — immigration restriction, trade protection, reluctance to enter into foreign entanglements, opposition to entitlement reform — have been more or less consistent for decades. What is equally true is that Mr. Trump has no hesitation in dropping a proposal, person or principle if he believes it will suit him. The programmatic details of Trumpism are fungible. The attitude behind it is not.

And Mr. Trump’s attitude is what came to define the conservative movement and the Republican Party. In 2017, Kellyanne Conway said that Mr. Trump’s presence at the Conservative Political Action Conference (CPAC) would rechristen the gathering as TPAC. She has not been proved wrong. Discussions of policy and the political horse race gave way to figuring out the best way to “own the libs” — a Trump specialty — and whatever controversy engaged Mr. Trump’s attention at the moment. For much of the right, unplacable and vocal resistance to cultural progressivism mattered more than experience, credentials or know-how.