WEST PALM BEACH, Fla. — The morning after North Korea launched a ballistic missile into the sea, apparently to test President Trump’s resolve in his first days in office, the new commander in chief wanted to make one thing very clear to the world: Mark Cuban, the billionaire Dallas Mavericks owner, was not smart enough to have his job.
“I know Mark Cuban well,” Mr. Trump said Sunday morning on Twitter, where he has 24.7 million followers and has found an even more prominent megaphone since he became president. “He backed me big-time but I wasn’t interested in taking all of his calls. He’s not smart enough to run for president!”
It was not clear what provoked the insult, although Mr. Cuban has recently been publicly critical of Mr. Trump. The president might have been reacting to a report on Sunday in The New York Post that White House aides view Mr. Cuban as a potential campaign rival in 2020, or to comments Mr. Cuban made to The Fort Worth Star-Telegram on Friday warning corporate executives to be careful in their dealings with Mr. Trump.
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But Mr. Trump’s put-down was only one in a long list of squabbles that the president has engaged in over the past week with individuals or groups that have aggrieved him. It offered a reminder three weeks into his tenure that even as he faces weighty problems, he is often preoccupied with the narrowest of gripes.
He swiped at Senator John McCain, Republican of Arizona, for criticizing the counterterrorism raid in Yemen that resulted in the death of a Navy SEAL. The raid has provoked anger in Yemen, where the government requested a review of the operation.
Mr. Trump said Mr. McCain’s critique “only emboldens the enemy,” and in a pair of postings on Twitter, he said that the senator, who is the chairman of the Armed Services Committee and was taken prisoner during the Vietnam War, has “been losing for so long he doesn’t know how to win anymore.”
The same day, the president took on Senator Richard Blumenthal, Democrat of Connecticut, for revealing that Judge Neil M. Gorsuch, Mr. Trump’s Supreme Court nominee, had called Mr. Trump’s attacks on judges “demoralizing” and “disheartening.” The comment was confirmed by aides who are shepherding Judge Gorsuch’s nomination, but Mr. Trump quickly branded Mr. Blumenthal a liar.
“Ask Senator Blumenthal about his Vietnam record that didn’t exist after years of saying it did,” Mr. Trump told reporters at the White House, referring to false statements that Mr. Blumenthal admitted to in 2010, after they were revealed.
Mr. Trump’s swing at Mr. Blumenthal was itself a function of yet another feud he has pursued, often in incendiary tones, against the judicial branch as it weighs the legality of his executive order banning travelers from seven predominantly Muslim countries.
It began this month when he called the Seattle judge who had blocked the directive a “so-called judge” who had made a “ridiculous” decision. He ratcheted up the insults during a speech to law enforcement officials from around the country, calling a hearing by a three-judge appeals court panel to review the stay “disgraceful” and comparing the intellect of the judges unfavorably with a poor student in high school.
And the president used Twitter twice last week to defend his daughter Ivanka, first targeting the department store chain Nordstrom — which had stopped carrying the apparel line bearing her name because of poor sales — for treating her “so unfairly.” Days later, Mr. Trump blamed journalists in a posting in which he expressed pride in Ms. Trump, whom he said had been “abused and treated so badly by the media.”
The White House is unapologetic about Mr. Trump’s outspoken style, even when it crosses the traditional lines observed by presidents of both parties, who have tended to avoid individual attacks on sitting senators, judges or individual companies, given the powers of the office. That Mr. Trump is willing, and even eager, to ignore those conventions, his aides say, is one reason his supporters adore him.
“Part of the reason the president got elected is because he speaks his mind,” Sean Spicer, the White House press secretary, said last week, questioned about Mr. Trump’s harsh words about members of the judicial branch. “He doesn’t hold it back, he’s authentic and he’s not going to sit back, I think, when he feels very passionately about something.”
Personal complaints and grievances have always weighed on, and sometimes motivated, American presidents in powerful ways, none more than Richard M. Nixon and Lyndon B. Johnson, said Matthew Dallek, a political historian.
“If you go back and listen to the tapes, they would talk privately with members of Congress or their staffs, and Nixon would say some pretty crazy things — about Jews, about people in the media who were out to get him — some of it was very petty, personal stuff,” Mr. Dallek said. “What is unusual is that President Trump is doing this publicly and it’s a near-daily occurrence, it’s multiple times a week.”
His habit of picking public fights is likely to appeal to the roughly 40 percent of voters who support Mr. Trump, but it may repel others whose backing he will need to govern effectively and, ultimately, to win re-election.
“It brings respect for the institution of the presidency way down, and it also plays into the narrative of all the millions of Americans who don’t see him as legitimate, because he’s using the power of the office now to attack individual citizens,” Mr. Dallek said.
In a post on Twitter responding to Mr. Trump’s insult on Sunday, Mr. Cuban shared a letter he had written to the president during his campaign last year, in which he advised Mr. Trump to drill down on policy specifics instead of improvising.
“I get that a big part of your base doesn’t care about issue details,” Mr. Cuban wrote to Mr. Trump, “but to be president, to be this close, you have to dig in and know your” facts.
Mr. Cuban’s initial response to the president’s sudden broadside was more concise: “Lol.”