Former President Donald J. Trump intensified his attacks against Nikki Haley on Friday, saying that she was not of presidential caliber and that, as a result, he was unlikely to choose her as his running mate.

“She is not presidential timber,” Mr. Trump said at a campaign event in Concord, N.H. “Now, when I say that, that probably means that she’s not going to be chosen as the vice president.”

Mr. Trump, known for off-the-cuff pronouncements that veer from his prepared remarks, continued by making it clear that his dismissal of Ms. Haley was not a fluke: “When you say certain things, it sort of takes them out of play, right?”

Mr. Trump is well known for excoriating someone in one breath, and then reversing himself when it becomes politically or otherwise advantageous to do so. But his stance toward Ms. Haley, whom he appointed as his ambassador to the United Nations, has hardened as the New Hampshire primary approaches.

Though Mr. Trump holds a wide lead over Ms. Haley in polls, she has narrowed the gap here in recent months. The Trump campaign is eyeing a decisive win in New Hampshire that could severely curtail her chances at winning the Republican nomination.

On Friday, he again accused her of being supported primarily by Democrats, saying that “leftists” were spending millions on advertisements so they could “flood your airwaves with Nikki propaganda.”

He acknowledged that she had done a “fine” job during his presidency. But he continued to insist that he had appointed her only so that he could elevate South Carolina’s lieutenant governor to her seat.

“All you need to know about Nikki Haley is that every corrupt and sinister group we’ve been fighting for the past seven years is on her side,” Mr. Trump said.

Mr. Trump’s remarks came as he scored a key endorsement from Senator Tim Scott of South Carolina, who shares a home state with Ms. Haley, and whom Ms. Haley appointed to the Senate.

Mr. Scott, whose endorsement was reported hours before the event, came onstage 25 minutes into Mr. Trump’s speech. He rattled off a litany of “needs” for the country, firing up the crowd and leading it in a spirited call and response that had the crowd shouting along with him: “We need Donald Trump.”

“We need a president who will close our southern border today,” Mr. Scott began. “We need Donald Trump.”

Mr. Scott, the nation’s highest-ranking elected Black Republican, concluded by saying, “We need a president who doesn’t see Black or white. We need a president who sees Americans as one American family.”

“That’s why,” Mr. Scott concluded, he had decided to endorse Mr. Trump.

Mr. Scott remains overwhelmingly popular with Republican voters, and his endorsement was sought by Ms. Haley. The former president seemed to delight in having won it.

“You know, I’m running against somebody from his state,” he said as he introduced Mr. Scott. “So it’s not that easy to do.”

The endorsement will add to speculation over Mr. Scott’s chances to serve as Mr. Trump’s running mate should he capture the Republican nomination. Mr. Trump said last week in Iowa that he had already settled on his choice, though neither he nor his campaign has provided details.

Another politician talked about as a potential running mate, Representative Elise Stefanik of New York, the No. 4 House Republican, spoke ahead of Mr. Trump and was to take part in another campaign event in Manchester, N.H., on Saturday.

Earlier, Ms. Stefanik — who endorsed Mr. Trump in November 2022 — told reporters that she would be honored to serve in a future Trump administration but declined to comment on any conversations she might have had with Mr. Trump about that possibility.

During her remarks, Ms. Stefanik recounted how she had grilled the presidents of three prestigious universities during a congressional hearing about antisemitism on college campuses.

Mr. Trump later praised her performance in that hearing. “Elise became very famous because she took on the heads of M.I.T. and Penn and Harvard,” he said, shortly before inviting her to the stage. “Wasn’t it beautiful?”

Maggie Haberman contributed reporting.

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