DONALD TRUMP’S primary victories Tuesday present the Republican Party with a stark choice. Should leaders unite behind Mr. Trump, who has collected the most delegates but may reach the convention in July without a nominating majority? Or should they do everything they can to deny him the nomination? On a political level, this may be a dilemma. As a moral question, it is straightforward. The mission of any responsible Republican should be to block a Trump nomination and election.

We do not take this position because we believe Mr. Trump is perilously wrong on the issues, although he is. His proposed tariff on Chinese imports could spark a trade war and global depression. His proposed tax plan would bankrupt the government while enriching his fellow multimillionaires. But policy proposals, however ill-formed and destructive, are not the crux of the danger.

No, Mr. Trump must be stopped because he presents a threat to American democracy. Mr. Trump resembles other strongmen throughout history who have achieved power by manipulating democratic processes. Their playbook includes a casual embrace of violence; a willingness to wield government powers against personal enemies; contempt for a free press; demonization of anyone who is not white and Christian; intimations of dark conspiracies; and the propagation of sweeping, ugly lies. Mr. Trump has championed torture and the murder of innocent relatives of suspected terrorists. He has flirted with the Ku Klux Klan and other white supremacists. He has libeled and stereotyped wide swaths of humanity, including Mexicans and Muslims. He considers himself exempt from the norms of democratic contests, such as the release of tax returns, policy papers, lists of advisers and other information that voters have a right to expect.

Does a respect for democracy require the Republican Party to anoint its leading vote-getter? Hardly. We are not advocating that rules be broken but that they be employed to maximum effect — to force a brokered convention and nominate a conservative candidate who respects the Constitution, or to defeat Mr. Trump in some other way. If Mr. Trump is attracting 40 percent of Republicans, who in turn represent about one-quarter of the country, that is a 10 percent slice of the population — hardly a mantle of legitimacy.

Republican front-runner Donald Trump spoke in Palm Beach, Fla., after primary voters took to the polls in five states: Florida, Illinois, Ohio, Missouri and North Carolina. (Reuters)

There are some Americans, Democrats in particular, who are happy to watch the Republican Party self-destruct with Mr. Trump at the helm. We cannot share in their equanimity. For one thing, though Hillary Clinton, the likely Democratic nominee, would be heavily favored, a Trump defeat is far from sure. For another, the country needs two healthy parties and, ideally, a contest of ideas and ideology — not a slugfest of insults and bigotry. Mr. Trump’s emergence already has done grave damage to American civility at home and prestige abroad. The cost of a Trump nomination would be far higher.

On Wednesday, Mr. Trump offered what was meant as an argument for his nomination. If he reaches the convention with a lead short of an outright majority, and then fails to win, “I think you’d have riots,” Mr. Trump said. “I think you’d have problems like you’ve never seen before. I think bad things would happen.”

A democrat disavows violence; a demagogue wields it as a threat. The Republican Party should recognize the difference and act on it before it is too late.