“You’re a traitor to the cause.”

In one form or another, that’s the charge most often made against so-called Never Trumpers, a group of which I consider myself an early and unofficial co-founder. The well-being of both the Republican Party and conservatism, according to this line of thinking, requires supporting Donald Trump. To be against him is to be an apostate.

Now it is certainly true that in the short run, and possibly in the long run, too, many of us no longer consider the Republican Party our political home. But for me, at least, a conservative approach to politics continues to lie at the core of my political being — and it is for that very reason that I believe even more strongly now, after what we have seen during Trump’s first term, that any true conservative should be appalled by the prospect of a second.

Put another way, to be anti-Trump is not to be anti-conservative; and to be pro-Trump is not to be pro-conservative.

That doesn’t mean that Mr. Trump doesn’t have any conservative policy successes he can claim. He does, though even here Mr. Trump’s record is not nearly as strong as his Republican defenders claim it is. From a conservative perspective, he’s gotten some things right and many things wrong.

The president is reshaping the judiciary in a conservative direction through his court appointments, but he has also given up on core conservative beliefs in limited government and responsible entitlement reform. He’s shredded federalism and embraced protectionism, both of which cut against conservative principles. It was also on Mr. Trump’s watch that, even before the pandemic hit, the United States set record annual deficits and exceeded $22 trillion in debt. (If Joe Biden becomes president, prepare for Republicans to rediscover a rhetorical commitment to fiscal discipline.)

The president’s conservative defenders point out that he has reduced unnecessary regulations on businesses, but they overlook the fact that he has proudly embraced crony capitalism and aggressively used the federal government to tilt the playing field and pick economic winners and losers.

Mr. Trump has promoted the conservative anti-abortion cause; he has also epically mismanaged a pandemic that has claimed more than 175,000 American lives. Before Mr. Trump, the Republican Party spoke out against so-called identity politics, yet today it embraces the worst form of white identity politics. The Republican Party once claimed to be home to constitutional conservatives, yet Mr. Trump has acted in ways that makes a mockery of our constitutional system of government, most recently through his use of executive orders to bypass Congress. (This is the kind of thing for which Republicans like Senator Ted Cruz and citizen Donald Trump excoriated President Barack Obama.)

Mr. Trump has increased spending on national defense, which has pleased conservatives, but he has also done more than any president in history to undermine NATO and the Atlantic alliance generally, which should not. The president has imposed tough sanctions on Iran even while engaging in bromances with Kim Jong-un of North Korea and Vladimir Putin of Russia and bonding with the authoritarian leaders Rodrigo Duterte of the Philippines and Recep Tayyip Erdogan of Turkey. He betrayed the Kurds and eroded our relationships with South Korea, Mexico, Canada, Germany and many other traditional American allies. He has also praised China’s forced internment of a million or more Uighurs as “exactly the right thing to do,” according to his former national security adviser, John Bolton.

Under Mr. Trump, then, the Republican Party is only incidentally conservative. At its core it is now ethnonationalist and populist, meaning that in its anti-establishment fervor it incites rather than refines public passions; it is increasingly antagonistic toward free markets, inward-looking and reactionary, hostile to diversity, pessimistic rather than optimistic, encased in cultural grievances, more interested in looking backward than forward.

But that is hardly where the erosion of conservatism ends.

The Republican Party once portrayed itself as the party of “family values,” insisting that character counts, especially in our political leaders, and most especially in our president. That was certainly the case during the Bill Clinton presidency. But today it has jettisoned all that, defending a rogue who paid hush money to a porn star while cheating on his third wife. Social conservatives once (rightly) warned against cultural decay, the coarsening of society, vulgarity and indecency. Yet Republicans now stand four-square behind a man who is uncaring and indecent.

The “party of ideas” is a phrase Daniel Patrick Moynihan used to describe the Republican Party in the early 1980s; today, large segments of the party are anti-intellectual, anti-science and dismissive of medical experts, to the point that it has turned wearing masks during a pandemic that’s spread by respiratory droplets into a “culture war” issue.

The party of law and order aggressively defends a president who is lawless. A party that for many years positioned itself as the defender of objective truth, a bulwark against subjectivism and ethical relativism, has as its leader a serially dishonest man who is engaged in a daily assault on reality.

During the 2016 campaign, Mr. Trump praised the conspiracy theorist Alex Jones, who spread the false narrative that the Sandy Hook school shooting was a hoax — and just last week the president praised QAnon, which Kevin Roose of The Times describes as “a sprawling set of internet conspiracy theories that allege, falsely, that the world is run by a cabal of Satan-worshiping pedophiles who are plotting against Mr. Trump while operating a global child sex-trafficking ring.” The lunacy and paranoia that was once on the fringe is now becoming more and more mainstream, which is hardly what one would expect to see in a serious, thoughtful conservative movement.

“Donald Trump is engaged in a disinformation campaign against his own country, which we’ve never seen before in an American president,” Jonathan Rauch, a senior fellow at the Brookings Institution who is writing a book about political attacks on truth, told me.

One conservative writer recently suggested to me that the Republican Party under Mr. Trump has become a dissident, anti-institutional party. There’s nothing that it really wants to conserve; the American right, under Mr. Trump, wants to burn things down. “It hates almost everything about contemporary America,” he said, adding that Trump supporters see the United States “as corrupt, degraded, and overtaken by forces they basically consider foreign in one way or another.”

Any attempt to rescue conservatism from the ashes, then, has to begin with the defeat of Donald Trump in November. If he wins a second term, whatever latent conservatism remains in the Republican Party will be extinguished. The redefinition of the Republican Party into the Trumpian Party will be complete and very difficult to undo. Conservatism as a political philosophy, as a political sensibility, will be homeless.

That would obviously be bad for those of us who are conservative; it would also be bad for the country. Conservatism at its best — conservatism properly understood — appreciates the complexity of human society, the role of civic institutions in the formation of human character, and the dangers of popular passions, mob mentalities and conspiratorial thinking. It places greater weight on human experience and practical wisdom than on the attachment to abstract theory and ideological purity that Ronald Reagan warned against in 1977.

There is — or at least there once was — such a thing as a conservative disposition. The great 20th-century British political philosopher Michael Oakeshott put it this way in his 1956 lecture “On Being a Conservative”:

The disposition to be conservative in respect of politics reflects a quite different view of the activity of governing. The man of this disposition understands it to be the business of a government not to inflame passion and give it new objects to feed upon, but to inject into the activities of already too passionate men an ingredient of moderation; to restrain, to deflate, to pacify and to reconcile; not to stoke the fires of desire, but to damp them down. And all this, not because passion is vice and moderation virtue, but because moderation is indispensable if passionate men are to escape being locked in an encounter of mutual frustration.

But conservatism is far more than merely a disposition. It is premised on certain beliefs, including the dignity and equality of individuals, while it is also undergirded by the idea that freedom is impossible without strong civic institutions and responsible personal behavior. It prizes prudence and considers gratitude a virtue. And because of its reverence for our Madisonian system of government — checks and balances, separation of powers — conservatism considers compromise part of our constitutional DNA.

Conservatism believes in limitations on the power of the state, it believes in the rule of law, it respects free markets as a generator of wealth and government and as a means to secure what the founders referred to as “unalienable rights.” It believes in defending the most vulnerable members of society, including the unborn. Understanding how easily a large, multiethnic nation can break into warring factions, conservatism finds ways to strengthen our bonds of affection, knowing that despite even deep differences we are not enemies but friends. It believes in objective truth while acknowledging the limitations of human reason and wisdom. We see through a glass darkly, knowing only in part, in the words of St. Paul.

Conservatism celebrates human excellence. It embraces change and reform as circumstances shift while always staying alert to the dangers of demagogues, fanatics and personality cults. And it stands against the destructive mind-set that seeks to destroy rather than to build up. “Rage and frenzy will pull down more in half an hour than prudence, deliberation, and foresight can build up in a hundred years,” as Edmund Burke famously put it.

Liberals should also hope for the revivification of a healthy conservatism. It would check some of the excesses of the Democratic Party, making it stronger and more responsible. In fact, some of the best laws passed by Congress in the past decades, on issues like Social Security, taxes and welfare, were the result of input from and compromise between the two parties.

But sound center-right parties also play a central role in well-functioning democracies everywhere. The Harvard political scientist Daniel Ziblatt, the author of “Conservative Parties and the Birth of Democracy,” concludes that democracy’s fate hinges on how conservative political parties — the historical defenders of power, wealth and privilege — recast themselves and cope with the rise of their own radical right.

“What conservatives look like is often a key determinant of how stable a democratic regime is,” according to Dr. Ziblatt.

As he told The Atlantic’s Uri Friedman, “When one looks around the world historically, at key moments, conservatives have been a hinge of history. Their reaction to forces of change shape whether or not a democracy survives.” In 19th-century England, Dr. Ziblatt argues, “there’s a positive case where conservatives played a critical role in helping support democracy.” On the flip side, one of the tasks in Weimar Germany was for the conservative party “to bind all of the right-wing forces to the regime.” It failed, with catastrophic consequences.

Which brings us back to Donald Trump. Conservatives who took a chance on Mr. Trump in 2016 can perhaps be excused — perhaps. He was a political novice running against an unpopular opponent who championed policies they disagreed with. He positioned himself as an agent of change in a nation longing for it. Many of his supporters hoped he would “grow in office,” that his aides would control him, that his worst tendencies would recede and some admirable ones would arise.

None of that happened.

This time around, Mr. Trump is fully known. No one believes he’ll change, and he has assured us that he has no intention of changing. His governing ineptitude and borderless corruption are undeniable; so, too, are his psychological and emotional disorders. He is not well, and the wreckage of his presidency is all around us.

Conservatives and Republicans therefore have to ask themselves: Are we willing to entrust our cause and our country to him for another term? Do we really want Mr. Trump’s venomous approach to politics and life to be even more deeply imprinted on the Republican Party? Isn’t it already poisoned enough for many young, nonwhite and suburban voters?

The detoxification of the Republican Party and the conservative cause therefore begins with the de-Trumpification of the Republican Party and the conservative cause. It is in the best interest of the country and conservatism to rid itself of the Trump presidency. Only then can the healing and rebuilding begin.

When it comes to his policy agenda, Joe Biden is no conservative. I wish he were. But despite efforts by Trump supporters to pretend otherwise, Joe Biden is not Bernie Sanders or Alexandria Ocasio-Cortez. Moreover, conservatism places a premium on prudence, human dignity, respect for the law and institutions, commitment to truth and reality, and a reasonable and reasoning governing temperament. In all of these respects, and others, Mr. Biden is more truly conservative than Mr. Trump.

That doesn’t mean that for conservatives there won’t be considerable costs to a Biden presidency; there will be. But it’s worth keeping in mind that sometimes the worst fears of what the other party will do never come to pass. For example, despite the fact that the Democratic Party supports a virtually unlimited right to abortion, the absolute number and the rate of abortions decreased steadily and significantly during the Obama-Biden administration. That is surely something to celebrate.

The 2020 matchup is hardly the one I’d prefer. No matter; it’s the one we have. Given that, there is one choice conservatives should make in the 2020 presidential race. And it’s not Donald J. Trump.

Peter Wehner, a senior fellow at the Ethics and Public Policy Center who served in the previous three Republican administrations, is a contributing Opinion writer and the author of “The Death of Politics: How to Heal Our Frayed Republic After Trump.”