What, then, might flip Mr. Trump’s removal from impossible to inevitable? The most likely possibility is also the most obvious: the collapse of his support among center-right Republicans who so far have wavered but not completely turned against him.

Whether this happens depends on future events, the most ominous of which would be the discovery of clear criminality by the president or those closest to him (including family members). Another inflection point might be an economic recession. A third might be Mr. Trump’s mismanagement of a crisis. A fourth would be the continued deterioration of the president’s behavior. (By most accounts the president feels less constrained than ever.) And yet another might be the prospect that he will lead his party to comprehensive defeat in 2020, especially if he is weakened by a primary challenge. We would be surprised if one or more of these developments did not occur, and a combination is easily within the bounds of probability.

Watergate showed how a president’s standing can cave in. Congressional Republicans supported and protected President Richard Nixon until the Watergate tapes provided irrefutable evidence of his wrongdoing. Then they withdrew their support and he resigned to avoid impeachment and conviction. There may be no single smoking-gun tape in Mr. Trump’s case, but the sheer weight of financial and ethical wrongdoing could become too much, even for many Republicans. And today’s Republican politicians, while more partisan than in Nixon’s day, remain acutely sensitive to public opinion. If some combination of criminality, incompetence or crisis moves the center-right against the president, his end could come quickly.

If that happens, Mr. Trump might step down to avoid impeachment, particularly if he were promised clemency for himself and his family. Short of outright resignation or removal, he could suffer enough defections so that he might announce he will not seek re-election. That would be a half-measure, but one that would allow the post-Trump conversation to begin. The Johnson-Nixon era provides precedents for all of these scenarios.

We understand the argument that the best result would be for voters, rather than the Republican Party, to do the job of removing Mr. Trump. But we believe this argument neglects an important reason that Mr. Trump’s removal by his party would be at least as healthy, democratically speaking: It would reinvigorate the idea that political parties exist not just as vehicles for politicians but also as protectors of vital democratic norms.

The most troubling — and from our point of view the most disappointing — development of the Trump era is not the president’s own election and subsequent behavior; it is the institutional corruption, weakness and self-betrayal of the Republican Party. The party has abandoned its core commitments to constitutional norms, to conservative principles and even to basic decency. It has allowed itself to be hijacked by a reality television star who is a pathological liar, emotionally unsteady and accountable only to himself. And Republicans have embraced presidential conduct that, had it been engaged in by a Democrat, they would have denounced as corrupt, incompetent and even treasonous.

We disagree with those who think that Mr. Trump’s removal by his own party would weaken democratic accountability; if anything, the opposite is true. The United States has only two major political parties, and it needs both to be healthy, rational and small-d democratic. They are our system’s most durable and accountable political institutions and they comprise its first and most important line of defense against political demagogues and conscience-free charlatans. By reasserting its institutional prerogatives — by setting limits to the depredations and recklessness it will accept — the Republican Party would be acting to deter hijackers in the future. In doing so, it would defend our democracy, not weaken it.