Sen. John McCain (R-Ariz.). (Saul Loeb/Agence France-Presse via Getty Images)

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WHEN U.S. Army Capt. Humayun Khan received his orders to go to Iraq in 2004, his parents asked him if there was some way he could avoid the deployment, because he had already done his service. He told them it was his “duty” — and doing it cost him his life. The concept of duty — fulfilling obligations because it is right — is something that Sen. John McCain (R-Ariz.), himself a distinguished war veteran, also understands. Perhaps that’s why he realized he had no choice but to rebuke Donald Trump for his abhorrent treatment of Capt. Khan’s parents.

“While our Party has bestowed upon him the nomination, it is not accompanied by unfettered license to defame those who are the best among us,” Mr. McCain said Monday in an extraordinary statement condemning his party’s presidential standard-bearer. By now — after Mr. Trump mocked Mr. McCain’s military service, called Mexican immigrants rapists and disparaged a federal judge for his heritage — nothing that he says or does should come as a surprise. Nonetheless, Mr. Trump’s comments in recent days about American Muslim parents Khizr and Ghazala Khan represent an abysmal new low.

Even in the face of mounting criticism from within his own party (among those finally finding some spine were New Hampshire Sen. Kelly Ayotte and Iowa Sen. Charles E. Grassley), Mr. Trump doubled down. “Mr. Khan, who does not know me, viciously attacked me from the stage of the DNC and is now all over T.V. doing the same — Nice!” he tweeted Monday. Anyone who has watched Mr. Khan or read the Post op-ed by his wife knows that, unlike Mr. Trump, the couple have been measured, dignified and within their rights as American citizens in expressing their thoughts and feelings. They are right to raise alarms about Mr. Trump’s call to exclude people like them and their son from this country on the basis of their religion. And they are right to question the empathy of someone who thinks nothing of denigrating a mother who lost a son in military service.

It is Mr. Trump’s own behavior that is cause for concern. His evident lack of any kind of self-control has ominous implications for how he would respond as president and commander in chief to real crises and emergencies. If his reaction to the Khans weren’t troubling enough, there is the separate problem of his ignorance of world events, something that was underscored over the weekend with his muddled account of Russia’s intervention in Ukraine.

Mr. McCain stopped short of withdrawing his endorsement of Mr. Trump, but the length and eloquence of his statement made clear just how appalling Mr. Trump’s actions are. Where Mr. McCain got it wrong was in challenging Mr. Trump “to set the example for what our country can and should represent.” As he should know by now, Mr. Trump doesn’t have a clue how to do that.