It’s customary, once a candidate has secured a party’s presidential nomination, for former rivals and party elders to line up behind the victor. Of course, some of their endorsements are more enthusiastic than others, but generally it isn’t just about promoting party unity; typically, leaders of a party will sincerely regard their own nominee as a superior alternative to the other party’s candidate.

But there is nothing typical about the impending nomination by the Republican Party of Donald J. Trump. Trump’s willful ignorance, hair-trigger temperament and bigoted comments about women and minorities absolve prominent Republicans of any obligation to fall into line even if they agree with him on the issues. (In fact, on several points he seems to depart from the party’s core positions.) Party loyalty is no excuse for supporting a manifestly unqualified and possibly dangerous candidate.