Donald Trump at the Family Leadership Summit in Ames, Iowa, on Saturday. (Daniel Acker/Bloomberg)

IT’S RARE that we dedicate our attention to marginal presidential candidates. But for a man as objectionable as Donald Trump, we will make an exception. Mr. Trump will flame out of the race, eventually. The big question is what effect his irresponsible demagoguery will have on Washington — and that poses a test to other, more credible candidates in both parties.

The best-case scenario is that his run will force those other would-be presidents to ask some bracing questions. Do they, too, at times and in their own ways, adopt Mr. Trump’s disfigured moral compass, encouraging voters to embrace anger, selfishness and suspicion? Can they instead be more like Mr. Trump’s latest victim, Arizona Sen. John McCain (R), a thoroughly honorable public servant who has resisted the urge to dish out the sort of poison that Mr. Trump offers to voters?

Over the past few days, Americans have seen a deep contrast in the exhibition of character. Following bizarre and offensive statements about how illegal immigrants are “rapists,” Mr. Trump belittled Mr. McCain’s war record, questioned his status as a war hero and sarcastically stated, “He’s a war hero because he was captured. I like people that weren’t captured.” Mr. McCain wasn’t just captured; he was brutally tortured by the North Vietnamese. Yet he refused special treatment or the early release he was offered.

Around that time, Mr. Trump was gallivanting around Manhattan, having obtained multiple draft deferments that allowed him to avoid military service. It is no surprise Mr. Trump can’t appreciate authentic heroism; his idols growing up appear to have been Broadway and Hollywood showmen, not people with Mr. McCain’s proven mettle.

There is Mr. McCain’s hard course of risking disfigurement and death for irreducible principles: honor, patriotism, personal integrity. Or there is Mr. Trump’s abandonment of any sense of shame in order to satisfy a common narcissism that equates grabbing attention with demonstrating quality. On the one hand is a politics that encourages Americans to be better than their basest instincts. On the other is extreme pandering that tickles populist nerve endings without regard to accuracy, fairness or consequences.

The Republican Party was privileged to have Mr. McCain as a presidential candidate, albeit a losing one. It is now cursed by association with Mr. Trump, a man whose foremost political talent is to reflect the worst instincts of American society.

Other presidential candidates should take note. Not because Mr. Trump’s approach, if honed and translated into base-rousing dog whistles, might not attract votes. But because no one wants to live in a country whose leaders have no conception of courage.