Republican presidential contender Donald Trump said on Dec. 7 that he was in favor of a '"total and complete" shutdown of Muslims entering the United States. (C-SPAN)

MOST CANDIDATES for the Republican presidential nomination, with an understanding of constitutional, democratic and social norms that Donald J. Trump utterly lacks, have denounced his outrageous call for a “total and complete shutdown” on Muslims entering the United States.

It is heartening that Mr. Trump’s opponents are finally condemning him in terms they would generally reserve for Democrats, but it also raises a critical question: If the GOP front-runner’s pronouncements are as lunatic and offensive as his rivals say — and they are — isn’t it incumbent on them to make clear they would oppose him if he were the party’s nominee?

The prospect of an open Republican split may send tremors down the spines of party strategists. They naturally fear an internecine war, a fractured party and maybe an independent Trump candidacy. But even those outcomes would cause less damage to their party and to the nation than uniting behind a candidate whose policies and rhetoric are morally, legally and pragmatically unconscionable — as they have now recognized.

Sen. Marco Rubio (R-Fla.) said Mr. Trump’s “habit of making offensive and outlandish statements will not bring us together.” New Jersey Gov. Chris Christie blasted Mr. Trump’s call for excluding Muslims as “ridiculous.” Former Florida governor Jeb Bush called the idea “unhinged.” House Speaker Paul D. Ryan (R-Wisc.) said Mr. Trump’s proposal “is not what this party stands for. And more importantly, it’s not what this country stands for.”

How can any of these men now contemplate endorsing the object of their obloquy?

Until now, party leaders and primary rivals have mostly dodged this question by dismissing Mr. Trump’s chances of winning the nomination. That’s no longer viable. Having stood atop the field in the polls for months, and lately having widened his considerable lead, Mr. Trump and his candidacy can no longer be laughed off as a publicity stunt. For responsible Republicans, the season of denial must end.

The plain truth is that a Trump presidency would not only fracture American society along ethnic, racial and, we now know, religious lines. It would also demolish American prestige on the world stage and alienate our most important allies. Think that’s an exaggeration? Then check with David Cameron, the Conservative British prime minister, who called Mr. Trump’s anti-Muslim hate-mongering “divisive, unhelpful and quite simply wrong.”

As it happens, on Monday, the same day Mr. Trump issued his latest incendiary call, authorities in Philadelphia began investigating an incident in which a severed pig’s head was tossed on a mosque’s doorstep, disrupting early-morning prayers. There has been a spike in similar incidents in recent weeks. This one, in the City of Brotherly Love, is a precursor of what would be an open season of such sectarian hate crimes in Mr. Trump’s America, were he elected president.

As Mr. Trump’s fellow Republican candidates now acknowledge, there is a real-world cost to a campaign that gains traction by spewing hatred, bigotry and rage. Criticizing Mr. Trump is no longer sufficient. It is time to say clearly he is anathema to the Republican Party, and to the nation.