“I’m just saying, I’m not a drinker,” he said. “It’s one of my only good traits. . . . I’ve never had alcohol. I’ve just — you know, for whatever reason. Can you imagine if I had what a mess I’d be? I’d be the world’s worst.”

I’ll raise a glass to that bit of introspection. The man often goes off on wild rhetorical benders fueled by nothing stronger than Diet Coke — something Trump was, at that very moment, proving anew.

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Sunday night’s agreement on a successor to the North American Free Trade Agreement had given Trump a legitimate reason to crow about a “very, very good,” “terrific,” “amazing,” “incredible” and “historic” deal. But he then opted to take questions on Monday, after aides had signaled he wouldn’t. Inevitably, he trampled his good news with a new rant about the Kavanaugh affair.

Trump, crying “witch hunt,” said that he’s with Kavanaugh “all the way,” and that his nominee has “been treated horribly” and “so viciously and so violently.”

He said one of Kavanaugh’s accusers “has very little credibility,” and cast doubt on his primary accuser, Christine Blasey Ford: “What day was it? Where was it? Do you know the location?”

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Once he got started, Trump couldn’t stop. He complained about “crap” he gets from the news media, saying journalists are “loco” (a word he used “because of the fact that we made a deal with Mexico”).

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He falsely said Sen. Richard Blumenthal (D-Conn.) claimed to be a “war hero” in Vietnam: “We call him ‘Da Nang Richard.’ ” He called Sen. Cory Booker (D-N.J.) a “horrible mayor ” and speculated that Sen. Dianne Feinstein (D-Calif.) leaked the Ford allegation “based on her very bad body language.”

And the president alleged he had seen an unnamed Democratic senator “in very, very bad situations, somewhat compromising.”

On and on he went — for 90 meandering minutes. Those arrayed behind him in the Rose Garden for the trade announcement fidgeted, bobbed and swayed.

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Somewhere in his disorderly mind, Trump had a notion that he needed to keep on the topic of trade. But his hectoring of reporters only distracted further.

When he called on ABC News’s Cecilia Vega, he added that “she’s shocked that I picked her, like in a state of shock.”

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Vega protested. “I’m not. Thank you, Mr. President.”

“I know you’re not thinking. You never do,” he said.

“I’m sorry?” replied Vega.

Just the kind of gratuitous insult of a woman that the White House needed amid the backlash over Kavanaugh’s alleged mistreatment of women.

Trump repeatedly rebuffed Vega and other reporters asking about Kavanaugh, saying he would take those questions later. To his credit, he did. And, when he did, he veered irrevocably off his script.

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Watching the president step all over his legitimate trade triumph, I couldn’t help but wonder how things might have turned out differently if he had self-discipline.

Strip away Trump’s zany hyperbole — that “NAFTA was perhaps the worst trade deal ever made” and that his revised trade deal creates a “manufacturing powerhouse” where factories never close and workers never lose jobs — and there is much to like about the new deal at first glance: potentially higher wages, tougher labor and environmental standards, a greater proportion of vehicles made in North America.

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What if instead of taking the alt-right, nationalist path, Trump had, from the beginning, devoted himself to infrastructure rebuilding and trade-pact renegotiation, both broadly popular ideas?

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What if he had negotiated a NAFTA replacement without sparking international chaos, calling Mexicans murderers and rapists and Canada’s prime minister “very dishonest and weak ”?

And what if he had been able to run his administration without the constant distraction of corruption, scandal, outrage, division and chaos?

We will never know. Even as Trump celebrated his agreement, he tossed in nonsense claims: U.S. Steel is “building eight or nine plants” and the Obama administration “said we’re not going to have manufacturing jobs anymore.” Soon he was talking about bump stocks and the border wall, “fake news,” his “landslide” victory in 2016 and 2018 midterm polls, and how “vocal” Kavanaugh was “about the fact that he likes beer” and the “difficulty” he had with alcohol — everything but what Trump was supposed to be talking about.

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Trump’s unpredictable careening from topic to topic is intoxicating to behold. That the president’s mind works this way is sobering.

Twitter: @Milbank