A Decisive but Shallow Debate Win for Harris Trump showed he isn’t up to the job. But her lack of substance won’t escape the voters’ notice.

He lost, she won, full stop.

Presidential debate split screen, Donald Trump and Kamala HarrisKamala Harris is a political athlete. And she can act—the amused, skeptical squint, the laughing tilt of the head, the hand on her chin. She was more interesting than Donald Trump, not only because she conveyed a greater air of dynamism but because she seemed interested in what was going on around her.

The two major headlines: First, Ms. Harris showed what she needed to show, that she is tough enough, bright enough, quick enough. People hadn’t really seen her tested. She had been elevated with mysterious speed in a drama whose facts we still don’t fully know. In the summer she made a good early impression with strong speeches and events. But she did all that on teleprompter. In the debate she wasn’t on teleprompter. She had to stand there and do it, and she did. Did she present herself as a plausible president? Yes.

Second, the incapacity of Mr. Trump. He was famously unable to portray her as outside the mainstream, but the news is he didn’t seem to try. He couldn’t prosecute his case because his sentences collapsed. He leaves words out, and he’ll refer to “he” and “them” and you’re not sure who he’s talking about. His mind has always pinballed, but Tuesday night the pinball machine seemed broken, like the flipper button wasn’t working and the launcher was clogged. He has been spoiled by his safe space, his rallies, where his weird free associations amuse the crowd and his non sequiturs are applauded as authenticity. That doesn’t work on a debate stage. It is strange he didn’t know this. And here is the news, for me. In the past it was possible to think he might make more sense next time. But I don’t think he can do better than this. I felt a lot of his supporters would be coming to terms with a deterioration in his ability to publicly present himself.

But here is an important sub-headline. Ms. Harris won shallowly. I mean not that she won on points, or that it was close—it wasn’t, she creamed him—but that she won while using prepared feints and sallies and pieces of stump speech, not by attempting to be more substantive or revealing. When you address questions in a straightforward way and reveal your thinking, you are showing respect. You’re showing you trust people to give you a fair hearing and make a measured decision. Voters can see it, and they appreciate it. They feel the absence of these things, too, and don’t like it.

She was often evasive, and full of clever and not-so-clever dodges. Trump supporters, and not only they, perceived a disparity in how the moderators treated the candidates. So did I. When Ms. Harris didn’t fully answer—even questions of major importance, such as immigration, the Afghanistan withdrawal, and her changes in political stands—they did not follow up or press her. I don’t remember a moment when anyone—including Mr. Trump—tried to pin her down. She got away with a lot of highly rehearsed glibness and often seemed slippery. Sometimes you have to slip and slide in politics but slipperiness doesn’t wear well.

Still, if you would be a Republican and president you must know how to ride with media predilections, how to be stern with your foe when the press won’t. And it’s hard to respect Mr. Trump for not calling the moderators on it in real time and then using it afterwards, like a blubbering baby, as an excuse for his failures.

We’ll see soon in polls the impact of her victory, whether it’s small or significant, and whether it changed much in the battleground states.

What should each candidate do now? I asked some Republican veterans, almost all of whom worked on George H.W. Bush’s 1988 campaign, after the debate. One said there is nothing for either camp to do but focus on turnout. “I think we are beyond changing minds, and I doubt the ‘debate’ did much to change any minds or significantly reduce the number of undecided. I think both sides are down to the ground game.”

Another agreed, saying that experience and data had taught him the value of reaching out and knocking on doors: “The best way to get out the vote is face-to-face contact.” Another said, “‘Let Trump be Trump’ isn’t where the electorate is at, and at this point is kinda self-defeating.” Mr. Trump should make sure his base maintains its excitement: “Do as many Fox and OAN town halls as possible.” A fourth old hound said the Harris campaign “should have a full-court press to get young women to vote, starting with sororities” in North Carolina and Georgia. He was thinking of Taylor Swift’s endorsement of Ms. Harris and its potential impact.

She should also do interviews—a lot.

Should there be a second debate? Absolutely. With 7½ weeks to go there’s plenty of time, and it would serve the public in that the more information the better; the better you know them the better. It could be good for both candidates. For Ms. Harris it would be a chance to appear more substantive in terms of policy and to nail down what progress she made Tuesday night. Whatever you like or could like, she could deepen. If she wins, that deepening would help her presidency. And clearly she’s not afraid. Mr. Trump could use another debate to try to recover from whatever he just lost, and to see if he can make a coherent case against the current administration, and for change. I don’t know if he has what it takes to achieve that. (Mr. Trump said on Thursday that he will refuse a second debate, so maybe he wonders too. But he not infrequently changes his mind.)

Taylor Swift in concertFinally, yes, it is amazing that Ms. Swift’s endorsement could change the outcome of the election but: America. We’ve been in love with our entertainers and celebrities since forever. If Rudolph Valentino had come out in support of Calvin Coolidge in 1924, his landslide would have been even bigger.

Ms. Swift’s statement, released at the end of the debate, was a little master class in how to cloak a dramatic move that might invite charges of hubris in an air of velvety modesty. She urged her fans to read up on the issues and do more political research. She timed her announcement so that it came at the exact moment everyone was consumed and distracted by the debate, thus taking any hard edges off its impact. She sweetly offered that she felt she had to make her stand clear because there was an artificial-intelligence thing out there in which she appears, falsely, to be endorsing Mr. Trump, and unfortunately he posted it to his site. So she’s just trying to clear things up and correct the record. It went out to her 284 million followers on Instagram.

Ms. Swift’s a real athlete too. And there is no way, in a 50/50 race, her decision won’t have impact.