For Mr. Cruz the lesson hit home with the damage, however small or significant, caused by Mr. Trump’s putting forward the issue of Mr. Cruz’s constitutional eligibility, due to birth in Canada, for the presidency. Mr. Trump was particularly devilish: He didn’t insist Mr. Cruz was ineligible but simply said, with an air of mock concern, that a President Cruz could bog the country down in years of court challenges. This was primo Doubt-Casting, aimed at giving potential Cruz supporters reservations.

As for Mrs. Clinton, she has clearly been rattled by Mr. Trump’s merciless resurrection of her alleged complicity in the sometimes brutal handling of women involved in her husband’s dramas. This reminds everyone of—and introduces young voters, who were children during the Gennifer Flowers through Monica Lewinsky stories to—the whole sordid underside of Clintonism. Mrs. Clinton clearly wasn’t expecting it, and she bobbled. She has never gone up against a competitor like Mr. Trump.

The second thing she and Mr. Cruz are both learning, I suspect, is something most people learn by their 20s: It matters what people think of you. It’s important that people have a high opinion of your essential integrity, trustworthiness and good faith. It matters that they like you. Mr. Cruz, when challenged by Mr. Trump, could have used some backup from prominent Republicans, but they didn’t throw him a lifeline. John McCain: “I am not a constitutional scholar on that, but I think it’s worth looking into.” You know why Mr. Cruz had no backup? Because almost no one who works with him likes him. They haven’t experienced him as a trustworthy person of good faith. They waited, as people do, for a chance to hurt him, and when they got it they did.