At their second debate, Donald Trump vowed to imprison Hillary Clinton if he becomes President. Photograph by Chip Somodevilla / Getty

"I didn’t think I’d say this," Donald Trump said to Hillary Clinton, as he took a couple of steps across the stage at the second Presidential debate, on Sunday night in St. Louis. "But I’m going to say it—and I hate to say it." At that point, just nineteen minutes in, it was already hard to imagine what might give Trump any qualms. He had already said that her record was "terrible" and "disgraceful," and that she "should be ashamed”; called Clinton's husband, Bill, the worst abuser of women "in the history of politics in this nation," and claimed that Hillary had "viciously" attacked women who had made allegations against him (three of those women were Trump’s guests at the debate); accused her of "laughing at the girl who was raped" by a man she had represented as a young lawyer (he'd brought her, too); accused her of being behind birtherism, which he himself had pushed; and objected when Clinton referred to Michelle Obama, who has been campaigning for her, as her friend—objected, it seemed, to the idea that Clinton could have any friends but Sidney Blumenthal ("he's another real winner that you have").

In those first nineteen minutes, Trump had also repeatedly insisted that a video that emerged last Friday—outtakes from "Access Hollywood," in which he told the host, Billy Bush, that because Trump was "a star" he could do whatever he wanted to women without their consent (Anderson Cooper, the moderator, paraphrased one line as "grabbing their genitals")—was an example of "locker-room talk." Trump made a vague expression of contrition, but with the air of a man looking over the moderators' shoulders for a sign pointing to said locker room. And he had claimed that Clinton had cheated in her primary victory over Senator Bernie Sanders. Referring to Sanders's endorsement of Clinton, Trump said, "I was so surprised to see him sign on with the devil." That was a few seconds before he sighed, overcame his supposed reluctance to speak harshly, and pushed American political discourse to even lower depths.

"If I win, I am going to instruct my Attorney General to get a special prosecutor to look into your situation," Trump said. "Because there has never been so many lies, so much deception. There has never been anything like it, and we’re going to have a special prosecutor." He had now turned toward Clinton, and was shaking his finger at her; at various points during the evening, which was set up as a town-hall meeting, he had paced behind her when she walked over to talk to audience members, shadowing her with all the subtlety of one of the agents in "Spy vs. Spy." At times, the oddness of their dance in that small space, and the unrestrained ugliness of Trump's attacks, meant that it didn't seem ridiculous to wonder if they might, actually, start shoving each other. But after he mentioned a special prosecutor, Clinton laughed. She knows all about special prosecutors, after all. The one Trump wanted would be for her e-mail, but there had been a number of them looking into all manner of things in her husband’s Administration—including some of the stories of the women Trump had brought with him—and she had made it through that. As Trump talked about the e-mails, and how she had used an "acid wash" on them, and said, for the second time, that she "should be ashamed," she stood up and composed her expression with a calm, pitying smile.

Martha Raddatz, the moderator, began to ask a follow-up, but Clinton interrupted her. "Let me just say, because everything he just said is absolutely false . . .”

"Oh, really?" Trump interrupted.

"I’m not surprised," Clinton continued, and explained that, given the number of lies Trump told, she would never get to talk about her actual policies if she spent all her time "fact-checking Donald." (That is a fair statement.) She mentioned a fact-checking hub on her Web site, and then said, "It’s just awfully good that someone with the temperament of Donald Trump is not in charge of the law in our country."

"Because you'd be in jail," Trump said. He drew out the last word—jay-ull—like an eight-year-old playground bully's taunt. A Presidential candidate saying that his opponent should be behind bars—and will be, if he has anything to do with it—may be new and shocking in a Presidential debate. But it is not new in this campaign. It is, indeed, one of the most regular features of Trumpism; "Lock her up" chants are heard at almost all his rallies. That doesn't make it less disturbing. The jail threat is jarring, in part, because it reflects what Trump understands the power of the President to be: he gets to lock her up. (Trump has a whole set of conspiracy theories based on the notion that President Obama is blackmailing Clinton with the threat that he will lock her up.) "Hillary for Prison" buttons are for sale on the Trump campaign Web site. And such threats are not new, sadly, in the world beyond America. The use of criminal law as a discretionary political tool is why, historically, some people have wanted to leave other countries to come here. Trump put an ugly smudge on that beacon.

In what may have been one of his more honest debate moments, Trump said, "I know nothing about Russia”—but he does seem to know that Putin gets to do that sort of locking-up thing to his opponents. Trump doesn't need to be a puppet of Putin to be a dangerous President. It is enough that he seeks to emulate his authoritarianism.

So when Trump’s supporters at the debate cheered at the mention of jail, it was not because he had come up with a fresh zinger but because he was reprising a favorite stock line. Cooper tried to quiet them—“Please do not applaud. You’re just wasting time”—and then Raddatz asked Clinton if she accepted the characterization by the director of the F.B.I., James Comey, of her handling of the e-mail as "extremely careless." (The two moderators worked together well.) Clinton responded by saying, as she has before, that having a private server was a mistake—“I'm not making any excuses”—and that there had been misinformation about the whole thing and that no harm had been done. She added that she had been good at keeping secrets during the mission to go after Osama bin Laden. But this is not the best area for her; she ended up squabbling with Trump about the number of e-mails she had deleted. "O.K., Donald. I know you’re into big diversion tonight, anything to avoid talking about your campaign and the way it’s exploding and the way Republicans are leaving you," she finally said.

That is true. Clinton won this debate; she was consistently more informed, she fought calmly and effectively under enormous pressure, and she spoke to American ideals, as in her answer to a Muslim-American woman who had asked about Islamophobia. She didn't simply say that the "Access Hollywood" video was disgusting; she connected it to Trump's larger mode of dealing with any number of groups and to a vision of the way he, and, in contrast, she, would govern. At the same time, she had moments that were less strong, as when she addressed her “deplorables” comment (she said that she was sorry she hadn’t concentrated on him, but didn’t quite reach out to the people who may have believed they were described), and when she was asked about a line in a transcript of one of her paid speeches (gleaned from a hacked e-mail) about how politicians needed to have a private and a public position. She responded, more or less, by recounting the incidents dramatized in the movie "Lincoln." It came across as her saying that Lincoln could be tactically secretive, too, which is true but was a rhetorically odd move. It set up Trump for an easy comeback: "Honest Abe, Honest Abe never lied. That’s the good thing. That’s the big difference between Abraham Lincoln and you." (The one thing that Trump can be relied on to know about American history is the nicknames.)