WITH SWEEPING wins in five East Coast states Tuesday, Donald Trump socked the forces within the Republican Party that have vowed never to support him. Picking up more than 100 pledged delegates, he brought himself closer to clinching the GOP nomination on the first ballot at July’s Republican National Convention, before most of the gains Sen. Ted Cruz (Tex.) has made in the party’s arcane delegate selection process could come into play. These facts in hand, Mr. Trump declared himself the presumptive GOP nominee.

If Mr. Trump were anything like a typical candidate, mathematical reality would result in a quick consolidation of the party behind him. In fact, that would have happened weeks ago. Now that the numbers are approaching prohibitive for “Never Trump” Republicans, pressure to rally around Mr. Trump will build. The excuses for making peace with the front-runner will be faulty but numerous — preserving party unity; avoiding a nasty convention fight; beating Hillary Clinton, the likely Democratic nominee. In his Tuesday night victory speech, Mr. Trump claimed — as usual without any corroborating details — that Republicans are already calling him seeking to mend fences.

They shouldn’t — because Mr. Trump is not a typical candidate. He is a unique threat to the Republican Party and to the country. The party should reject him as a nominee, using any and all legitimate means to do so. Principled Republicans must make a concerted stand in Indiana and California, the two states left to vote that could keep Mr. Trump short of the 1,237 delegates he needs to clinch the nomination. As long as there is opportunity to resist his rise, those who recognize what he is must take it. This is not because beating Mr. Trump is a likely outcome. It is because, morally, there is no other option. New math does not change the ethical calculation.

Mr. Trump degrades people, serially insulting women, Latinos, Muslims, immigrants, Jews and others. He erodes the discourse, frequently and flagrantly lying about things such as whether “scores” of terrorists have recently entered the United States as migrants — one of numerous false claims he made in a speech on foreign policy Wednesday. He proposes undermining foundational civic institutions such as the free press. He shows contempt for the separation of powers by threatening the speaker of the House. Where his policy agenda is not thin, it is scary, such as his call to ban Muslims from entering the United States. In short, he should inspire fear that someone so lacking in judgment and restraint could acquire the powers of the presidency.

If Mr. Trump nevertheless got the GOP nomination, many Republicans would face a similar dilemma to the one they face now. Once again, appeals to party unity and victory in November would offer them excuses to ignore the simple moral calculation required to recognize Mr. Trump’s unacceptability. In fact, they would have at least three ways to avoid making this error — voting for Ms. Clinton, running a third-party conservative candidate or refusing to vote at all. None of these options promises to put a Republican in the White House. But each would at least spare some Republicans the moral stain of association with Mr. Trump.