House Speaker Paul D. Ryan in New Berlin, Wis., on Feb. 19. (Greg Moore/Associated Press)

EVEN AS Donald Trump attempted to tone himself down Tuesday night, he could not help underlining why he is such a menace. “I’m going to get along with [House Speaker Paul D. Ryan],” Mr. Trump said. “And if I don’t, he’s going to have to pay a big price.”

Mr. Ryan (Wis.) should make a down payment right now. He and other Republican leaders ought to make clear that they will not “get along” with a man who, from the beginning of his campaign, has profited from voter prejudice and hatred. They should reject his authoritarian assault on American democracy.

Unfortunately, most GOP leaders have so far proved unwilling to take this moral leap. True, Mr. Ryan has not been totally silent as Mr. Trump has risen, most recently condemning the billionaire’s reluctance to condemn the Ku Klux Klan on national television. “If a person wants to be the nominee of the Republican Party, there can be no evasion and no games,” Mr. Ryan said Tuesday. “They must reject any group or cause that is built on bigotry.”

How about Mr. Trump’s campaign? “This party does not prey on people’s prejudices,” Mr. Ryan insisted. But it is on the path to nominating a man who has done just that. If Mr. Trump wins the nomination, what would the speaker do? “My plan is to support the nominee,” he said Tuesday.

“I hope this is the last time I need to speak out on this race,” Mr. Ryan also said. Barring a stunning transformation by Mr. Trump, that won’t be the case. The front-runner’s appeals to racism, attacks on the media and threats against those who oppose him are unlikely to cease. If Mr. Ryan wants to be on the right side of the history that may be written about this presidential race, he must condemn Mr. Trump clearly and comprehensively. The same goes for every other Republican leader. The way for them to prove that their statements of principle have real-world meaning is to pledge not to vote for Mr. Trump.

Maybe that would ultimately split the party. Maybe not. Mr. Trump has taken only 34 percent of the cumulative popular vote so far. Some Republicans believe that the case against him as a sketchy businessman and lifelong fraudster still has not been made in the earnest, sustained way required to reach voters. If so, they must try harder to make it. There are two weeks before big winner-take-all state primaries that could cement Mr. Trump’s lead. This is not a time for soft criticisms, veiled warnings and shortsighted partisanship.

Despite what some Republicans appear to believe, Hillary Clinton is not more dangerous than Mr. Trump. No job is worth the moral stain that would come from embracing him. No party is worth saving at the expense of the country.

Republicans should listen to retiring Rep. E. Scott Rigell (R-Va.). “My love for our country eclipses my loyalty to our party,” Rigell recently wrote, “and to live with a clear conscience I will not support a nominee so lacking in the judgment, temperament and character needed to be our nation’s commander-in-chief.”