Mr. Trump’s newfound class was on display this week, in reaction to Saturday’s state GOP convention in Colorado. Lacking almost any organization, Team Trump was smoked. Sen. Ted Cruz took all 34 delegates. In response, Mr. Trump tweeted, “The people of Colorado had their vote taken away from them” and warned, “This will not be allowed!”

Actually, it will be. The state Republican executive committee voted unanimously last August to select delegates through a convention, not a primary or caucus. Mr. Trump, running initially as a lark, failed to organize in states like Colorado. Now he demands that the rules be changed because he didn’t prepare and lost.

Mr. Trump has been running a vanity campaign. His looming victory in next week’s New York primary may discourage him from doing so, but at some point, maybe he should run a real one.