But no rising figure in the G.O.P. is likely to consider sacrificing their career to make a protest or start an intellectual debate — leaving the task to retirees and elder statesmen, to a Flake or a Bob Corker or a John Kasich or even a Mitt Romney. And there is every reason to think that most of them would demur as well, and that the inevitable challenger will look more like the third party challengers of 2016 — an Evan McMullin or a Gary Johnson, or some foolhardy NeverTrump pundit drafted into the lists.

However: It would also be a mistake to assume that the present environment will necessarily persist — that because Trump has held Republicans thus far there is nothing that could shake his 38 percent support. George W. Bush was bonded to the G.O.P. base by partisanship and wartime leadership and shared religious values and his approval still sank to the 20s when events turned sharply against his presidency. Trump does not have nearly as far to fall as Bush, some of his support is soft, he has already lost seven points off his January 2016 approval ratings, and neither war nor economic crisis have really tested him as yet.

What might weaken him further? An economic slowdown. An overseas debacle too significant to be explained away by Fox’s talking heads. The indictment of a family member. A botched nomination the next time a Supreme Court vacancy occurs. Something strange and Trumpy, that cannot yet be foreseen.

Partisanship and incumbency being what they are, even with approval ratings in the twenties Trump would still be the favorite to win his party’s nomination. But the possibilities for a challenger would widen, and so might the list of possible contenders.

Such a contender, though, would need to be shrewd as well as bold. Various obvious lines of attack against Trump are ill-suited for a primary assault. He’s not an ideological conservative was tried and found conspicuously wanting in ’16. Pious attacks on his moral failings, however justified, are likely to also feel like indictments of his voters — whose backing a primary challenger would need to win. And absent something dispositive from Robert Mueller, going all-in on the Russia scandals would link a challenger too closely to the Democrats.

Instead, a primary campaign would need to be waged more in sorrow than in anger, accusing Trump of broken promises, lamenting his administration’s inability to legislate, and promising to carry on certain parts of his agenda (judges, above all) but with more competence and tact. The challenger would need to criticize Trump from the ideological right on some issues, but also reach out to more populist Republicans, especially from Pew’s “Market Skeptic” category, by asking why his infrastructure bill never happened, why his tax cuts haven’t done more for the middle class, why he isn’t doing more to stop outsourcing and bring back the coal industry and so on down the list of (yes, always-implausible) broken promises. And then instead of accusing Trump of being a racist or misogynist or authoritarian (as true as those accusations may be), the challenger would simply lament that after pledging to drain the morass of Washington the president let his administration be taken over by swamp creatures.