Donald Trump and Hillary Clinton at the first Presidential debate, on Monday, on Long Island. Photograph by Mandel Ngan / AFP / Getty

“Words matter when you run for President,” Hillary Clinton said toward the end of Monday night's happening at Hofstra University, on Long Island. Clinton was criticizing Donald Trump for his loose language regarding America's allies in Asia, but she could have been summing up the lopsided debate, which saw her doing virtually everything she needed to do while Trump indicted himself with his own words.

As anybody familiar with Clinton's career could have predicted, she was extremely well prepared for her first debate against Trump. After finding an opening to voice the key theme of her campaign early on—“We have to build an economy that works for everyone, not just those at the top”—she spoke confidently and with precision more or less throughout. Although some of her attack lines sounded rehearsed—“Trumped-up trickle-down” was one that she repeated—they were still effective. She didn't say anything that will come back to haunt her, and her only really awkward moments came when her opponent attacked her for reversing her position on the Trans-Pacific Partnership.

Trump, on the other hand, had evidently been telling the truth, for once, when, in the lead-up to Monday night, he said he didn't believe in spending a lot of time on debate prep. His spontaneity and direct language played to his advantage in some of the debate’s early exchanges. But, as the night wore on, and as the discussions got more detailed, his lack of respect for the format got him into all sorts of trouble.

First, there were his errors of omission. Trump’s harsh stance on immigration and his depiction of Clinton and other professional politicians as puppets of corporate interests have both helped propel his campaign to this point. But on Monday, speaking before an enormous national audience, he barely mentioned the wall he wants to build across the U.S. border with Mexico, and he didn't bring up Clinton's ties to monied interests at all.

Then there was the damage done by the things he did say. At one point, Lester Holt, the moderator, asked Trump about his refusal to release his tax returns, a subject that Trump must have known would come up. He replied by saying that he would release his returns “when she”—Clinton—“releases her thirty-three thousand e-mails that have been deleted.” Perhaps Trump thought he was being smart with this answer, but it only gave Clinton a chance to respond, which she did with relish.

“So you've got to ask yourself: Why won't he release his tax returns?” she said, seizing the moment. “Maybe he's not as rich as he says he is . . . maybe he's not as charitable as he claims to be.” Then Clinton raised another theory, one that I and others have written about: “Or maybe he doesn't want the American people, all of you watching tonight, to know that he's paid nothing in federal taxes.”

Here, you’d expect the target of the attack to sense the danger. Evidently, Trump didn't. Having interrupted Clinton during most of her previous answers, he did so again. “That makes me smart,” he said.

Even on Twitter, where people were pulling apart Trump's words with the relish of a class of third graders dissecting a worm, it took a few seconds for this statement to sink in. Had he really just boasted that he didn't pay any federal taxes? Indeed, he had.

And that wasn't the end of it. After Clinton pointed out the implications of Trump's boast—“So if he's paid zero, that means zero for troops, zero for vets, zero for schools or health”—Holt changed the subject and asked about her e-mails. She said what she has said before: that setting up a private e-mail server while serving as Secretary of State was a mistake, for which she accepted responsibility.

A better, less cavalier debater than Trump would have seized on this opportunity to do what Clinton had just done to him: pull the scab off and poke around. He could, for example, have brought up the F.B.I. director James Comey's pointed criticisms of Clinton's handling of her e-mail. But he did nothing of the sort. Instead, after making a few comments about how one of the technicians who set up the server took the Fifth, refusing to answer questions from Congress, Trump changed the subject back to his own gaping wound, saying, “As far as my tax returns, you don't learn that much from tax returns. That I can tell you.”

There followed another brief peroration about how rich he was, and how it was “about time that this country had somebody running it that has an idea about money." Thanks to know-nothing politicians like Clinton and President Obama, Trump said, the United States is now twenty trillion dollars in debt. "We have a country that needs new roads, new tunnels, new bridges, new airports, new schools, new hospitals. And we don't have the money, because it's been squandered on so many of your ideas." Clinton, one imagined, could barely wait to get in her riposte. It was worth waiting for: "And maybe because you haven't paid any federal income tax for a lot of years,” she said.

To this viewer, at least, Trump seemed oblivious to the harm he was wreaking upon himself. Instead, he seemed almost to be revelling in it. Perhaps Trump believes that soccer moms in suburban Philadelphia and janitors in Ohio will warm to him for his ability to avoid paying taxes on what he referred to as his "tremendous income." Perhaps he also thinks the voters will reward him for stiffing his workers, cheating his suppliers, and reneging on his businesses debts. That is about the only way to rationalize his insouciant answers when Clinton, again showing her preparedness, started to delve into some of the less salubrious aspects of his business career, of which there are many.

"I have met a lot of the people who were stiffed by you and your businesses, Donald," Clinton said, baldly but calmly. "I've met dishwashers, painters, architects, glass installers, marble installers, drapery installers—like my dad was—who you refused to pay when they finished the work that you asked them to do. We have an architect in the audience who designed one of your club houses at one of your golf courses. It's a beautiful facility. It immediately was put to use. And you wouldn't pay what the man needed to be paid, what he was charging you to do."

Rather than letting Clinton deliver her spiel and then moving on to another question, Trump interrupted again. "Maybe he didn't do a good job, and I was unsatisfied with his work," he said. Clinton wasn't put off. After asking whether all the people Trump had cheated weren't owed an apology, she brought up her father again, saying that she was relieved he "never did business with you. He provided a good middle-class life for us, but the people he worked for, he expected the bargain to be kept on both sides." Then Clinton reminded the audience that Trump businesses have entered bankruptcy six times, adding, "There are a lot of great businesspeople that have never taken bankruptcy once."

Trump simply couldn't zip it and move on. "I built an unbelievable company, some of the greatest assets anywhere in the world," he said. Then, referring to the bankruptcies, he added, "Four times . . . we used certain laws that are there. And when Secretary Clinton talks about people that didn't get paid, first of all, they did get paid a lot, but [I’ve] taken advantage of the laws of the nation. Now, if you want to change the laws, you've been there a long time; change the laws. But I take advantage of the laws of the nation because I'm running a company. My obligation right now is to do well for myself, my family, my employees, for my companies. And that's what I do."

He couldn't have said it any more clearly: that's what he does. It was good of him to put in on the record, and in a Presidential debate, no less. And it was Clinton who lured him into doing it.

_Read more about the first debate: Amy Davidson on how Trump failed to bully Clinton, Benjamin Wallace-Wells on how Clinton turned Trump into Mitt Romney, and Jill Lepore on how Presidential debates have evolved on television.

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