Floundering in his attempts to wield political power while lacking a political office, Donald Trump looks increasingly like a stray orange hair to be flicked off the nation’s sleeve. His residual power, which he must use or lose, is to influence his party’s selection of candidates for state and federal offices. This is, however, perilous because he has the power of influence only if he is perceived to have it. That perception will dissipate if his interventions in Republican primaries continue to be unimpressive. So, Trump must try to emulate the protagonist of “A Connecticut Yankee in King Arthur’s Court.”...