Republican lawmakers have seen the Trump disaster coming for a while now. They simply have no clue what to do about it.

A couple of months ago — before we learned that Donald Trump Jr. wanted to spend quality time with people he believed represented the Russian government, before the president publicly humiliated his attorney general and was abandoned by top business executives, before he claimed “some very fine people” were marching in Charlottesville, Va., alongside neo-Nazis and white supremacists — a Republican member of Congress I spoke with called the president a “child king,” a “self-pitying fool.”

Even then, the words that came to mind when some congressional Republicans described the president were “incompetent” and “unfit.” There were concerns about his emotional stability. “There’s now a realization this isn’t going to change,” one top Republican aide on Capitol Hill said. Yet there is the simultaneous realization, as a House member told me when talking about Republicans in their home districts, that “we’re never going to have a majority of people against him.”

Maybe, but for now this presents Republican members of Congress who are privately alarmed by Mr. Trump with a predicament. Regardless of what he does, a vast majority of his core supporters are sticking with him. A recent Monmouth University poll found that of the 41 percent of Americans who currently approve of the job he’s doing, 61 percent said they cannot see Mr. Trump doing anything that would make them disapprove of him. Mr. Trump was on to something when he said in January 2016, “I could stand in the middle of Fifth Avenue and shoot somebody and I wouldn’t lose voters.”

The political problem facing Republicans is that Mr. Trump’s presidency is a wreck. His agenda is dead in the water. A special counsel is overseeing an investigation of his campaign. The West Wing is dysfunctional. And President Trump is deeply unpopular with most Americans.

A new Washington Post-ABC News poll illustrates the dilemma Republican politicians face. It found that 28 percent of polled voters say they approved of Mr. Trump’s response to Charlottesville. But among Republican voters, the figure was 62 percent, while 72 percent of conservative Republicans approved.

The more offensive Mr. Trump is to the rest of America, the more popular he becomes with his core supporters. One policy example: At a recent rally in Phoenix, the president said he was willing to shut down the government over the question of funding for a border wall, which most of his base favors but only about a third of all Americans want.

Much of this mess is of the Republican Party’s own making. Let’s not forget that Mr. Trump’s political rise began with his promulgation of the racist conspiracy theory that President Obama was not a natural-born American citizen. The Trump presidency is the result of years of destructive mental habits and moral decay. So there’s no easy solution for responsible Republicans. But there is a step they have to take.

They need to accept, finally, the reality — evident from the moment he declared his candidacy — that Mr. Trump is unfit to govern. He will prove unable to salvage his presidency. As the failures pile up, he’ll act in an even more erratic fashion.

The mental hurdle Republicans have to clear is that in important respects the interests of the Republican Party and those of Donald Trump no longer align. The party has to highlight ways in which it can separate itself from the president.

So far the response of many Republican leaders to Mr. Trump’s offenses has been silence or at most veiled, timid criticism. The effect is to rile up Trump supporters and Mr. Trump himself without rallying opposition to him. It’s the worst of all worlds.

What’s required now is a comprehensive, consistent case by Republican leaders at the state and national levels that signals their opposition to the moral ugliness and intellectual incoherence of Mr. Trump. Rather than standing by the president, they should consider themselves liberated and offer a constructive, humane and appealing alternative to him. They need to think in terms of a shadow government during the Trump era, with the elevation of alternative leaders on a range of matters.

This approach involves risk and may not work. It will certainly provoke an angry response from the Breitbart-alt-right-talk-radio part of the party. So be it. Republicans who don’t share Mr. Trump’s approach have to hope that his imploding presidency has created an opening to offer a profoundly different vision of America, one that is based on opportunity, openness, mobility and inclusion.

This requires a new intellectual infrastructure to address what may prove to be one of the largest economic disruptions in history. People in positions of influence need to make arguments on behalf of principles and ideas that have for too long gone undefended. They must appeal to moral idealism. And the party needs leaders who will fight with as much passionate intensity for their cause as Mr. Trump fights for his — which is simply himself. There’s no shortcut to forging a separate Republican identity during the Trump presidency. Half-measures and fainthearted opposition are certain to fail.

If Republicans need more encouragement to break with Mr. Trump, they might note that the president, who has no institutional or party loyalty, is positioning himself as a critic not just of Democrats but also of Republicans. During his rally in Arizona, he went out of his way to attack both of that state’s Republican senators, including one battling brain cancer. He followed that up with tweets attacking the House speaker, Paul D. Ryan, the Senate majority leader, Mitch McConnell, and other Republican lawmakers.

A confrontation is inevitable. The alternative is to continue to further tie the fate and the reputation of the Republican Party to a president who seems destined for epic failure and whose words stir the hearts of white supremacists.

We are well past the point where equivocations are defensible, and we’re nearly past the point where a moral reconstitution is possible. The damage Mr. Trump has inflicted on the Republican Party is already enormous. If the party doesn’t make a clean break with him, it will be generational.

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