Washington — Republicans who are thinking about opposing President Trump in the 2020 primaries are facing the hardest of political choices.

Toppling a sitting president of your own party is a maneuver with the highest degree of difficulty. The most relevant historical model is probably Eugene McCarthy’s race against Lyndon Johnson in 1968, which helped convince a politically wounded president to withdraw. But McCarthy had a clear policy handhold — opposition to an increasingly unpopular war — and appealed to a discontented element of his party.

What are the handholds for a challenger to Trump? Economic conservatives are generally happy with the 2017 tax cut. Social conservatives are generally satisfied with Trump’s judicial nominees (and should be). Foreign policy conservatives are generally not pleased with Trump’s sabotage of alliances, his compulsive personal diplomacy and his abdication of leadership in promoting American values. But the Republican foreign policy establishment was almost uniformly opposed to Trump the last time around, and it mattered not at all.

So why undertake this difficult, perhaps thankless political task?

First, no political moment is permanent. After a particularly damaging new administration scandal (not unlikely) or a severe economic downturn, a hopeless quest might suddenly seem like remarkable political foresight. Or not. But no alternative to Trump can benefit from changing circumstances if he or she doesn’t run in the first place. Fortune favors the slightly irrational.

Second, the Robert Mueller report and a string of congressional investigations could destabilize Trump’s personality in escalating and disturbing ways. The president could move against important institutions, or against the separation of powers, in a manner that causes a serious portion of the Republican electorate to reconsider its blind support. I am not holding my breath, but who could judge this impossible?

Third, even in the absence of a policy handhold, there are elements within the GOP that seem open to a counter-Trump message. In one recent poll, 16 percent of Republicans prefer for Trump to be a one-term president. On the evidence of the midterm, this discontent skews young and female. Many of these voters, presumably, are less focused on Trump’s tax policy, and more on his racism and misogyny. It is at least a place to start.

At this stage of the 2020 campaign, the Republican case against Trump is not mainly about policy or ideology (though it could be eventually). It is not primarily about his ignorance and refusal to learn and improve at his job (though that is concerning). The main Republican argument against Trump is this: He is a person of horrible character who corrupts everyone around him, undermines essential social standards and is branding his party with an image of bigotry that will last a generation.

The problem with Democrats making this argument about Trump’s character is simple. To abandon the president in favor of a Democrat, Republican voters are forced, not just to value public character, but to value public character above conservative economic policy and above the appointment of conservative judges. And — thought it pains me to say it — not many Republicans place that much weight on matters of character. They will take Trump plus Justice Brett Kavanaugh over any Democratic of unimpeachable integrity. If, however, any of the serious Republican prospects — Nikki Haley, Jeff Flake, Bob Corker, Mitt Romney or John Kasich — run against Trump, Republican primary voters will face the challenge: Why not conservative policy and public character?

This is the main reason that some Republican on that list (or some talented candidate still unknown) must run. There needs to be an alternative focus of intellectual energy and moral leadership in America’s party of the right. This is what a presidential campaign — successful or not — can accomplish. To those who say it is useless to protest the direction of the GOP, a campaign embodies the reply: “Well, I protest anyway.” To those who say that traditional conservatism is a lost cause, it represents the answer: “Not to me.” To those who claim that the effort can’t succeed, it says: “Let’s deserve success first, then see where honorable effort leads us.”

At some point, there must be a limit to political calculation. History has an honor roll of those who show seemingly futile courage. Someone on the American right must contend that racism and sexism violate the promise at the heart of America. Someone must be offended when national ideals are debased by cruelty and corruption. Someone must be willing to defy good political sense in a great political cause.

— Michael Gerson is a columnist for Washington Post Writers Group.