AS DONALD Trump was building a campaign on lies, bigotry, insults, fearmongering and unreason, a few Republican leaders of apparent principle offered some resistance. Foremost among them was House Speaker Paul D. Ryan (R-Wis.). In March, Mr. Ryan insisted that “all of us as leaders can hold ourselves to the highest standards of integrity and decency” and that “we shouldn’t accept ugliness as the norm.”

On Thursday Mr. Ryan capitulated to ugliness. It was a sad day for the speaker, for his party and for all Americans who hoped that some Republican leaders would have the fortitude to put principle over partisanship, job security or the forlorn fantasy that Mr. Trump will advance a traditional GOP agenda.

Explaining his belated endorsement of Mr. Trump in a home-state newspaper, the speaker said that conversations with the presumptive Republican presidential nominee have reassured him. Mr. Trump will help turn House GOP ideas into law, Mr. Ryan said, in a way that a President Hillary Clinton would not.

This is fanciful, as Mr. Ryan must understand. Judging by his wild swings of position over the years, Mr. Trump does not believe in much of anything. The convictions that he does hold — against free trade and U.S. leadership abroad, for dividing the nation by religion and ethnicity — are antithetical to the principles Mr. Ryan has said guide him. Having secured the nomination without Mr. Ryan’s help, a President Trump certainly would not feel bound by any assurances that Mr. Ryan believes he has heard from the candidate.

Paul Ryan endorsed presumptive Republican presidential nominee Donald Trump on June 2, but the two haven't always seen eye-to-eye. (Peter Stevenson/The Washington Post)

“That’s the thing about politics,” Mr. Ryan said a while back. “We think of it in terms of this vote or that election. But it can be so much more than that. Politics can be a battle of ideas, not insults. It can be about solutions. It can be about making a difference. It can be about always striving to do better. That’s what it can be and what it should be.”

Now Mr. Ryan has endorsed a man whose “solutions” include banning Muslims from entering the country, who casts aspersions on judges because of their ethnicity, who mocks people with disabilities, who lies repeatedly, who would muzzle the free press. Each one of these is disqualifying — particularly for anyone who believes in conducting the nation’s politics in a constructive, reasonable manner or who claims to have the long-term interests of the nation, rather than a short-term win at the ballot box or in Congress, in mind.

Following Mr. Ryan’s endorsement, some insisted that the speaker had little choice. This is false. “My dad used to say, ‘If you’re not a part of the solution, you’re a part of the problem,’ ” Mr. Ryan said in March. When he has a comparable conversation with his children, how will Mr. Ryan explain the decision he made in this campaign?