Despite President Trump’s insistence on Tuesday morning that his national security adviser, Michael Waltz, “has learned a lesson” after inadvertently including the editor of The Atlantic in a cabinet-level chat session on Signal, speculation continues to build about Mr. Waltz’s job security.

Mr. Trump vigorously defended Mr. Waltz in front of television cameras during an event a few hours later, saying he should not have to apologize for the breach.

“That man is a very good man, right there, that you criticized,” Mr. Trump said, pointing to Mr. Waltz after a reporter asked if the president would order practices to be changed. “So he’s a very good man, and he will continue to do a good job. In addition to him, we had very good people in that meeting, and those people have done a very, very effective job.”

Most of the Republican Party leaped to Mr. Waltz’s defense, seeking to blame the news media for the uproar.

But in interviews, several close allies of the president characterized the national security adviser’s standing as precarious, more so than it already was when The New York Times reported on his uneasy status over a week ago. Those who discussed Trump administration views on Mr. Waltz did so on the condition of anonymity to speak candidly. His fate, they say, rests on Mr. Trump’s caprices, with several competing factors coming into play.

On the one hand, it is Mr. Trump’s nature to defy a media firestorm rather than try to quell it by offering up a sacrificial lamb. He parted from this tendency at the beginning of his first administration when he fired his national security adviser, Michael T. Flynn, for not divulging his encounters with Russian officials to the F.B.I. According to one adviser from that era, Mr. Trump soon regretted that act of acquiescence.

This time around, according to several people who have spoken to Mr. Trump over the first two months of his term, he wants to avoid firing people because of the narrative of chaos that it will quickly engender. Once he starts firing people, one person familiar with his thinking said, it will be very hard to draw a line if problems arise with other aides down the line. And Mr. Trump has appeared increasingly more concerned with holding his perceived enemies at bay than anything else.

Mr. Waltz also benefits from a much closer relationship to the president than Mr. Flynn had. As a Republican congressman from 2019 until his current appointment, Mr. Waltz had been an unflagging defender throughout Mr. Trump’s political and legal travails. He spent much of last year campaigning for Mr. Trump, often traveling aboard the candidate’s private plane. He aggressively questioned the director of the U.S. Secret Service at a hearing after an assassination attempt on Mr. Trump at a rally near Butler, Pa., and became a defender of Mr. Trump against the agency.

Perhaps more significantly, Mr. Waltz frequently served as a surrogate for the Trump campaign on Fox News, thereby passing the eyeball test for a president-elect who prefers his senior aides to be telegenic.

But Mr. Waltz has now given Mr. Trump reason to second-guess his loyalty, two people familiar with the matter suggested. The detail that Jeffrey Goldberg, the editor of The Atlantic, appeared to be in Mr. Waltz’s list of contacts to begin with — and therefore mistaken for another “JG” to be invited into the Signal group chat — has sent up alarms among the president’s allies, according to people familiar with their thinking.

In The American Conservative, a founding editor, Scott McConnell, wrote Tuesday, “I don’t see how National Security Adviser Mike Waltz organizing a group chat with The Atlantic’s Jeffrey Goldberg goes away without Waltz’s resignation.”

In The Atlantic article, Mr. Goldberg recounted that Mr. Waltz had sent him a connection request on Signal on March 11, adding that he “didn’t find it particularly strange that he might be reaching out to me.” Asked about the Signal fiasco in a news conference with Mr. Trump Tuesday, Mr. Waltz described Mr. Goldberg as someone “I’ve never met, don’t know, never communicated with.” In an interview for this article, Mr. Goldberg said that he had met Mr. Waltz a few years ago at two events but had never interviewed him.

Ironically, it was Mr. Waltz’s familiarity with members of the U.S. foreign policy establishment, including Mr. Goldberg, that provided relief to some quarters after he was named to second Trump administration. A former Green Beret and four-time recipient of the Bronze Star, Mr. Waltz had served in the national security apparatus for the Bush and Obama administrations before working for a defense contracting firm and then running for Congress.

“Mike’s exceptionally well-rounded,” said Peter Bergen, an author and national security analyst who wrote the foreword to one of Mr. Waltz’s books. “I saw it as an inspired choice on Trump’s part.”

Others saw Mr. Waltz as a curious selection. An avowed hawk, he staunchly defended the wars in Iraq and Afghanistan in his 2014 book “Warrior Diplomat.” In a podcast interview in 2021, he warned that withdrawing U.S. troops from the latter, as Mr. Trump had proposed doing, was “the best way to cause another 9/11 to happen.” Mr. Waltz instead advocated a sustained troop presence like the one that has been in Colombia — “a great model” — for over three decades. Such views have caused Mr. Waltz to be branded a “neocon” in right-wing circles.

Many of those who have heralded Mr. Waltz’s capabilities now find themselves at pains to explain his breach of security protocol. At the news conference on Tuesday, Mr. Trump reiterated that Mr. Waltz was “a very good man” and that attacks on him were “very unfair.” But some of the president’s allies have speculated that this appraisal could change if his national security adviser is increasingly viewed with ridicule.

Those who have known Mr. Trump throughout the years point to a striking constant: While he has a high tolerance for lightning rods, he has a very low one for laughingstocks.

Maggie Haberman contributed reporting.