WASHINGTON — Less than a year ago, President Trump was savaging Kim Jong-un, the North Korean leader, as a “madman” and murderer of his own people, branding him as “Little Rocket Man” for his nuclear testing exploits and poking fun at his portly stature.
Early Thursday morning, a jubilant Mr. Trump described how Mr. Kim had been “excellent” to three American prisoners he had agreed to release from a prison in North Korea, and “nice” to free them so early — a “wonderful thing” that showed Mr. Kim’s desire to end his country’s isolation. The comments came two weeks after Mr. Trump praised Mr. Kim as “very honorable” in discussions about a coming summit meeting.
The head-snapping rhetorical turn has accompanied a major shift in the dynamic between the United States and North Korea as Mr. Trump and Mr. Kim prepare for the unpredecented meeting next month in Singapore. It also underscores the president’s black-and-white worldview — a stark formulation in which there is good or evil, friend or enemy, and not much in between — that has opened him to criticism that he has been too quick to embrace a brutal leader as a worthy negotiating partner.
“We’re starting off on a new footing,” Mr. Trump said at Joint Base Andrews, in Maryland, where he had arranged for a showy middle-of-the-night arrival for the prisoners to be broadcast live on television. He added, “There’s never been a relationship like this.”
Critics pounced on the remarks as evidence that the president was in the process of being duped and outplayed by Mr. Kim.
“We can’t be fooled into giving the North Korean regime credit for returning Americans that never should have been detained in the first place,” said Senator Chuck Schumer of New York, the Democratic leader. “It is so troubling to hear President Trump say that Kim Jong-un treated the Americans excellently.”
Over-the-top messaging, both positive and negative, is nothing new for Mr. Trump, whose 1987 book “The Art of the Deal” argued that “bravado” was crucial to salesmanship.
“People want to believe that something is the biggest and the greatest and the most spectacular,” the book says. “I call it truthful hyperbole.”
As president, Mr. Trump has married that style with a distinctly personal approach to foreign policy, in which he appears to have utmost confidence in his ability to win over his counterparts around the world with flattery, shows of respect and plays to ego — all things that he relishes himself.
White House officials said this week that Mr. Trump had made it clear that he was unwilling to offer any concessions or negotiate for the release of the prisoners, and expressed confidence that Mr. Kim would respect that position and do the right thing anyway.
“It’s not unusual in diplomacy for people to be yelling at each other in one moment and singing kumbaya in the next, but what makes this so dramatic is that the president went further in both directions than others have,” said Peter D. Feaver, a political scientist at Duke University who was a national security official under George W. Bush. “I think that’s a reflection of his belief that we have to break the standard playbook here — we’ve tried everything else with North Korea and it hasn’t worked, so why not try something different?”
Few dispute that Mr. Trump has shattered the norms of diplomatic communication when it comes to his language about Mr. Kim — both positive and negative — and raised the stakes for the nuclear negotiations along the way.
“He has gone from extremely negative comments on Kim designed to frighten him into coming to the negotiating table to, now, extremely flattering comments designed to make him conclude a deal,” said Sue Mi Terry, a former C.I.A. analyst who is a senior fellow for Korea at the Center for Strategic and International Studies. “It’s just so over the top. No president has ever spoken in either way.”
The love-hate dynamic started during Mr. Trump’s candidacy in 2015, when he described Mr. Kim as “this maniac sitting there” with nuclear weapons who had to be dealt with. Once Mr. Trump became president, the insults continued. In 2016, Mr. Trump called the North Korean leader a “bad dude.” He became more heated in his denunciations over the summer after Mr. Kim tested a long-range missile that appeared capable of hitting the United States.
“Does this guy have anything better to do with his life?” the president wrote on Twitter. Weeks later, he vowed to rain down “fire and fury like the world has never seen” if North Korea continued to threaten the United States.
Mr. Trump brought his streetfighter-style words to the United Nations the next month with another threat to Mr. Kim. “Rocket Man is on a suicide mission for himself and for his regime,” he said, a line that he embellished during a campaign rally in Alabama not long after, branding the North Korean leader “Little Rocket Man.”
His advisers privately cringed at the talk, concerned that the president’s warnings and insults could escalate tensions with Mr. Kim, who is known to be thin-skinned, to the point of precipitating a nuclear crisis.
Now, the concern is in the other direction, that Mr. Trump is raising expectations for his ability to achieve a breakthrough with Mr. Kim that may not materialize, based purely on a personal rapport that he is overstating.
“The problem is that this gives a sense that he’s so willing to conclude a grand bargain with Kim that he’ll do anything,” Ms. Terry said of Mr. Trump. “I don’t think he can help it.”
Michael D’Antonio, a Trump biographer, said that from a young age, Mr. Trump had become accustomed to using grandiose words to threaten or flatter, usually to sell something, oftentimes himself.
“I actually think that these guys speak the same language,” Mr. D’Antonio said of Mr. Trump and Mr. Kim, adding that the summit meeting would bring together two leaders who are supremely focused on their own images, intolerant of slights and addicted to flattery.
“So we’re going to have a narcissist duet in Singapore, with these two guys, maybe the only two guys on Earth who know this song so well,” Mr. D’Antonio said. “And they’re going to sing it to each other.”