At first glance, in the wake of the roars of approval President Trump received from Republican lawmakers during his State of the Union address on Tuesday night, it may seem fanciful to think the president might be forced out of office by members of his own party. At second glance, too. Yet sometimes things that seem impossible one moment become inevitable the next.

We believe that Mr. Trump’s strength among Republicans is more precarious than it appears. For reasons both substantial and practical, we believe his disgorgement by Republicans can happen, might happen — and should happen. Contrary to conventional wisdom, removal by his party would be as healthy for America’s democracy as his removal by the voters, perhaps more so.

Being sane, we understand why the prospect of Mr. Trump’s being forced to resign or face impeachment and conviction before the end of his term is unlikely. His losing a renomination fight seems only slightly more likely. After all, his ejection depends on a significant part of the Republican Party turning against him — and right now it belongs to him.

Mr. Trump’s Gallup approval rating among Republicans is almost 90 percent and has never dipped significantly below 80 percent. His followers defenestrate Republicans who defy or repudiate him. In the recent midterm elections, Mr. Trump and his backers further consolidated their grip on the party, even as they lost their grip on the House of Representatives.

But that is hardly the whole story. Recent developments should deeply worry Republicans, starting with those disastrous midterms. The Republican Party may have held on to the Senate, but Democrats now control the House of Representatives because they won more congressional seats than they had since the post-Watergate tsunami of 1974. They gained seven governorships and nearly 350 state legislature seats. According to exit polls, Democrats improved over their 2014 midterm showing by six or more percentage points among men, women, married voters, unmarried voters, whites, Hispanics, Asians, voters under 30, voters over 59, moderates, independents, urbanites and voters with college degrees.

In other words, Republicans lost significant ground among everyone except Mr. Trump’s core base of rural, evangelical and “noncollege” supporters (and even among them, the Republican margin shrank a bit). This happened with unemployment lower than at any time since 1969 and with Republican turnout at its highest level in a century.

Mr. Trump’s hard-core base is large enough to dominate the Republican Party, at least for now, but it is not large enough to dominate the country. In the long run, a third or so of the country cannot effectively govern the other two-thirds with an unpopular agenda and a Twitter account. Mr. Trump will almost surely achieve less legislatively in the second half of his term than he did in the first, when Republicans controlled both branches of Congress — and even then their record was not impressive.

In short, by consolidating behind Mr. Trump, the Republican Party is isolating and alienating itself from the broader public. Indeed, the Trump paradox is that his support deepens among his most persistent admirers even as it erodes everywhere else. As a result, Mr. Trump headed into his third-year State of the Union message with the second-lowest approval rating in history, despite a roaring economy. (Ronald Reagan, who bested Mr. Trump for this dubious honor in 1983, spoke at the nadir of a deep recession.)

Meanwhile, chaos is consuming the Trump administration. The president, cowed by his base, engineered a very unpopular government shutdown for which most people held him (and his party) responsible. He finally agreed to a deal to reopen the government, but only on terms dictated to him by House Speaker Nancy Pelosi, to the dismay of some of his most prominent right-wing supporters. In December, his mercurial decision-making drove his widely respected defense secretary to quit in protest. Scandals and corruption besiege the president on every side. His administration is being investigated by a special counsel, by the Southern District of New York and soon by House Democrats armed with subpoena power. The president’s behavior is becoming more erratic and bizarre, and his own aides have confided that he is “unhinged.”

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 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 it has embraced presidential conduct that, if engaged in by a Democrat, it would have been 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.

In any event, the Republican gamble that the party can ride out the Trump era without suffering tremendous damage is looking worse every day. As Republican lawmakers have privately told us and others, they know Mr. Trump will not change. The incontestability of his psychological defects and character flaws has finally sunk in. What remains to be done is for Republicans to prevent what many of them privately know is quite likely for their party if Mr. Trump remains their leader: a crash landing.

In that sense, Mr. Trump’s presidency has become to the Republican Party what Vietnam was to President Lyndon Johnson. By 1965, Johnson saw Vietnam for the unwinnable quagmire that it was, but he feared and ultimately bowed to the short-term consequences of withdrawing. “It’s like being in an airplane and I have to choose between crashing the plane or jumping out,” he told his wife. “I do not have a parachute.” We know today that Johnson made the wrong decision.

Increasingly, it is dawning on Republicans that they are making the same mistake. But they do have a parachute, one named Mike Pence. The vice president would continue many of Mr. Trump’s policies, if that’s what they want, but potentially without all the dysfunction, a result that conservatives could live with and that the voters could judge for themselves in 2020.

In a recent column about the sudden possibility that Britain would change its mind about Brexit, the economist Anatole Kaletsky remarked: “In times of political turmoil, events can move from impossible to inevitable without even passing through improbable.” The same is true of Trexit.

Of course, Mr. Trump’s exit is a long shot. In democracies, sick political parties usually need years in the wilderness before they can heal. We have not talked ourselves into being confident, or even particularly optimistic, that the Republican Party will treat its own fever. But if there is one thing that the age of Trump has clarified, it is that “unimaginable” and “impossible” are not at all the same thing.

Peter Wehner, a senior fellow at the Ethics and Public Policy Center, served in the last three Republican administrations and is a New York Times contributing opinion writer.

Jonathan Rauch is a senior fellow at the Brookings Institution and the author of “Political Realism: How Hacks, Machines, Big Money and Back-Room Deals Can Strengthen American Democracy.”