ON SUNDAY, ABC’s George Stephanopoulos asked Republican National Committee Chairman Reince Priebus whether the party would back Donald Trump should he win the GOP nomination. “Yes, we will support the nominee,” the Republican chairman replied. “To me, it’s a no-brainer.” Mr. Stephanopoulos asked if a Trump nomination would split the party. “Winning is the antidote to a lot of things,” Mr. Priebus responded.

Winning can quiet many complaints, it is true. But it cannot and will not be an antidote to the moral poison of Mr. Trump’s campaign. Party leaders who support and celebrate his victory will be accomplices to an attack on the fundamental values of American democracy. Winning will not wash away the stain.

Mr. Trump’s campaign is based on suspicion and unreason. He revels in policy proposals that make no sense. He stirs bigotry against Muslims, Hispanics, Jews, people with disabilities and more. He demeans war heroes. His latest turn is indirectly questioning Sen. Marco Rubio’s (R-Fla.) eligibility to be president, a suspicion rooted in pure prejudice.

Mr. Trump appears to have turned illogic into a virtue for his supporters, asking his audiences, “Who’s going to pay for the wall?” The reply is as enthusiastic as it is bizarre: “Mexico!” How might that happen? The answer, or rather the absence of one, is irrelevant to the candidate. How will he respond when, having reached the Oval Office, his simplistic promises proved unachievable, he encounters opposition in the form of legitimate checks and balances from the courts and Congress? Which ethnic group will he pick on to explain away his failures to deliver? What actions would he take to distract people from his lack of substance?

Like many GOP leaders, Mr. Priebus has shown that he knows that Mr. Trump is a problem. He condemned Mr. Trump’s plan to ban Muslims from entering the United States. But, also like many top Republicans, the party chairman has nevertheless given Mr. Trump a wide berth to run a flamboyant insult of a campaign.

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There are several pretexts Republican officials might offer to avoid giving Mr. Trump the public thrashing he deserves: condemnations from “establishment” politicians might only make him stronger; the results of the GOP nominating process deserve some respect, and Republicans must abide by the rules; maybe Mr. Trump would beat the Democrats in November.

None of these excuse silence. Particularly not the third. The argument that any Republican would be better than any Democrat is a depressing reflection of irrational partisanship. Mr. Priebus and everyone else “leading” the GOP are Americans before they are Republicans. They should act like it.