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.