Three days before the New Hampshire primary, former President Donald J. Trump brought a handful of special guests onstage at his largest rally in the state this year: a slew of South Carolina Republicans.

In a show of force meant to undermine his main rival for the Republican presidential nomination, Nikki Haley, the former governor of South Carolina, the out-of-state visitors told thousands of people in an arena in Manchester, N.H., that they had made the trip to urge New Hampshire voters to reject Ms. Haley and unite behind Mr. Trump, in order to bring a swift end to the Republican primary.

“We’re here for one reason,” Gov. Henry McMaster, who succeeded Ms. Haley in the governor’s mansion, said in a thick Southern drawl. He then quoted the Spice Girls: “What we’re here to do is tell you what we want in South Carolina, what we really, really want.”

In another election cycle, the scene might have qualified as bizarre: a handful of out-of-towners whose primary election will not take place for another month telling New Hampshire residents, who pride themselves on their “live free or die” independence, to heed their call to end the race for the Republican nomination before it even reaches their state.

And the crowd was willing to listen. Mr. McMaster concluded his speech by crying: “New Hampshire is for Trump. South Carolina is too. We’ll see you at the finish line!” A man in the crowd shouted back, “God bless South Carolina!”

The traveling pack of South Carolina Republicans offered strong evidence of the challenge that Ms. Haley and Gov. Ron DeSantis of Florida face as they try to get a foothold in the race after Mr. Trump’s decisive win in Iowa. The Trump campaign hopes the display will add to his momentum before Tuesday’s voting, and highlight Ms. Haley’s lack of support in her home state.

Mr. Trump and his team are eager to force both Ms. Haley and Mr. DeSantis from the race before the South Carolina primary on Feb. 24, hoping to avert what could otherwise be an expensive fight for delegates lasting through March.

“We’re going to be there in three weeks,” Mr. Trump said. “So you know what I’m doing? I’m kissing ass.”

Since his landslide victory in Iowa, Mr. Trump has made the case that overtaking him is so unlikely that his rivals should suspend their campaigns so that he and the Republican Party can focus on defeating President Biden in November.

“We’re going to have another election on Tuesday, but after that, when Trump wins overwhelmingly, we’ve got to get behind him,” Representative William Timmons, one of the South Carolina leaders who came to New Hampshire, said after the rally.

He and Mr. McMaster were joined at the rally by South Carolina’s lieutenant governor, Pamela Evette; Alan Wilson, the attorney general; Curtis Loftis, the treasurer; Murrell Smith, the speaker of the Statehouse; and two other congressmen from the state, Joe Wilson and Russell Fry.

The guest appearances could humiliate Ms. Haley and further undermine her case for the nomination by illustrating how isolated she appears to be in her own state, where the Republican primary will be held on Feb. 24. And the South Carolina group backing Mr. Trump on Saturday will follow the state’s junior senator, Tim Scott, who endorsed Mr. Trump in Concord, N.H., on Friday.

Ms. Haley is not without support in South Carolina, where she served as governor from 2011 to 2017, when Mr. Trump appointed her as ambassador to the United Nations. She has the backing of Representative Ralph Norman and Katon Dawson, a former state Republican chairman, along with a small number of South Carolina legislators.

When asked at a campaign stop in Peterborough, N.H., about Mr. McMaster’s campaigning with Mr. Trump, Ms. Haley fired back: “I’m sorry, is that the person I ran against for governor and beat? Just check it.” Ms. Haley defeated Mr. McMaster in a 2010 primary, and he then promptly endorsed her.

She also suggested that her inability to win high-profile supporters both in her home state and in Washington came from her willingness to pressure state lawmakers and veto their pet projects when she was governor, along with her criticism of Congress on the campaign trail.

“If Donald Trump wants all his politicians, and they all want to go to him, they can have at it,” Ms. Haley said. “But that’s everything I’m fighting against.”

Ms. Haley’s path to the nomination likely hangs on a victory or a close second-place finish in New Hampshire, where independent voters make up 40 percent of the electorate. Though Mr. Trump maintains a large lead in the polls, Ms. Haley has narrowed that lead recently, effectively making the primary a two-person race in the state.

But Ms. Haley would need to follow a strong performance in New Hampshire with another one in South Carolina, where Mr. Trump enjoys a large and loyal following. He leads in South Carolina polls by a wide margin and can also count on support there from Senator Lindsey Graham, a close ally.

Campaigning on Saturday in Myrtle Beach, S.C., Mr. DeSantis was asked about South Carolina elected officials’ endorsing Mr. Trump. “Iowa Republican leadership lined up behind me, and we came in second,” Mr. DeSantis told reporters. “So I think there’s a limit to what the leadership can do.”

As the New Hampshire primary nears, Mr. Trump has increasingly escalated and sharpened his attacks on Ms. Haley. He now often argues that while she did an adequate job in his administration, she does not have what it takes to lead on her own.

“She is not presidential timber,” Mr. Trump said bluntly in Concord on Friday.

Mr. Trump has also repeatedly tried to backpedal on his past praise of Ms. Haley, claiming frequently that he appointed her ambassador to the United Nations only to clear the way for Mr. McMaster to become governor.

Jazmine Ulloa and Nicholas Nehamas contributed reporting.