THE REPUBLICAN presidential race, the headlines tell us, suddenly turned civil and substantive this week. When the candidates faced off in their last debate before Tuesday’s crucial primaries in Florida and Ohio, they did not trade petty insults or discuss body parts. Instead, Donald Trump merely stated that “there is tremendous hate” among “large portions” of the world’s 1.6 billion Muslims. “We better solve the problem before it’s too late,” Mr. Trump said.

A debate is not civil when it includes this kind of ignorant stereotyping. It is not substantive when such rank prejudice earns inadequate protest from the others onstage. In fact, it is a measure of how crude Mr. Trump has made GOP politics that the front-runner’s hateful spewing seems increasingly normal.

It should not be normal. It should be unacceptable. Republican leaders should condemn Mr. Trump as often as he makes these outrageous statements, and with equal force. Instead, Republican National Committee Chairman Reince Priebus warmed up Thursday’s debate crowd by declaring that any of the GOP candidates would be “a world better” than Hillary Clinton. Sen. Ted Cruz (Tex.) said Mr. Trump’s rhetoric is “incendiary” but then argued that his policies would not be hard-line enough. Other Republican leaders remain mostly silent, hiding behind the excuse that attacking Mr. Trump might only help him. They must ask themselves whether they want to be complicit by omission in his poisoning of American political life.

True, when asked to comment Thursday night, Sen. Marco Rubio (Fla.) pushed back — a bit. “Presidents can’t just say anything they want,” Mr. Rubio said, warning that Muslims who might have been friendly toward Americans abroad would be less so if U.S. leaders sounded like Mr. Trump. Mr. Rubio also noted that Muslims serve faithfully in the U.S. armed forces. “Islam has a major problem on its hands,” Mr. Rubio concluded, but “we are going to have to work with people of the Muslim faith.”

Yet the response to Mr. Trump must be bigger than arguing that his rhetoric is unfair to a relative handful of Muslims in the armed forces and alienating to foreigners the United States needs. Mr. Trump’s incitements are unacceptable first and foremost because they are morally repulsive. Writing off a modern religion based on the actions of a tiny minority of its adherents dehumanizes people who have just as much claim to individual dignity as any white, Christian American. The concept that all people deserve to be judged based not on their creed or skin color but, rather, on their individual attitudes and actions is a fundamental American value. The great American experiment in pluralistic democracy, in which allegiance to common civil values, not common ethnic or religious origins, will fail without this underlying concept. Mr. Trump is corrosive to the nation’s foundational principles.

According to debate moderator Stephen Dinan of the Washington Times, Sen. Jeff Flake (R-Ariz.) responded to Mr. Trump by saying, “Republicans are better than this.” In fact, the jury is still out.