I supported the Republican in dozens of articles, radio and TV appearances, even as conservative friends and colleagues said I had to be kidding. As early as September 2015, I wrote that Mr. Trump was “the most serious candidate in the race.” Critics of the pro-Trump blog and then the nonprofit journal that I founded accused us of attempting to “understand Trump better than he understands himself.” I hoped that was the case. I saw the decline in this country — its weak economy and frayed social fabric — and I thought Mr. Trump’s willingness to move past partisan stalemates could begin a process of renewal.

It is now clear that my optimism was unfounded. I can’t stand by this disgraceful administration any longer, and I would urge anyone who once supported him as I did to stop defending the 45th president.

Far from making America great again, Mr. Trump has betrayed the foundations of our common citizenship. And his actions are jeopardizing any prospect of enacting an agenda that might restore the promise of American life.