New Hampshire Gov. Sununu endorses Haley as she hopes to catch up to Trump

New Hampshire Gov. Chris Sununu endorsed Nikki Haley in the 2024 Republican primary on Tuesday night -- a shake-up in a key early voting state where Haley and a few others have only weeks left to try and catch front-runner Donald Trump.

Sununu made the announcement in Manchester, New Hampshire, to a standing-room-only crowd with a handful of hopeful attendees gathered in heated tents outside in below-freezing temperatures.

"Let's get this thing. We are all in on Nikki Haley," he yelled after saying he'd been asked about his support by an older woman and regular Haley attendee when entering the event.

"To see her out there, to see her connecting with folks, to feel that momentum -- it is real and it is tangible," Sununu said of Haley. "Now, there's a lot of work to do."

As he alluded to, Haley faces real polling challenges.

According to 538's average, she is currently at nearly 19% support in New Hampshire; Trump is at about 45%.

An endorsement from Sununu, a vocal Trump critic and popular figure in the state that prides itself on independent-minded voters, could help close that gap ahead of the state's Jan. 23 primary.

"We gotta be able to move forward," he said on Tuesday night, echoing Haley's argument that new leaders -- not Trump -- are needed.

After exchanging a warm embrace with Sununu, Haley joked to the crowd, "I think all Manchester is here. The question is: Did they come for you or they come for me?"

She went on to call Sununu's support "about as rock solid of an endorsement as we could get." Some voters say it may sway them, too.

"I have a lot of faith in Sununu," said Christy Carlson, an undecided voter from Merrimack, New Hampshire. "So whoever he does get behind that endorsement means a lot to me. That'll be big."

Linda Webb from Wolfeboro, New Hampshire, agreed: "I have an enormous amount of respect for the governor, and I am hopeful." She said that she also hopes Sununu appears on the 2024 ticket somewhere with someone.

Back in June, Sununu opted out of throwing his own hat in the race for a White House bid because he felt a crowded field helped Trump. Since then, various Republican hopefuls have essentially been auditioning with Sununu for his endorsement and he narrowed his list down to three governors in the past few weeks: Haley, Florida Gov. Ron DeSantis and former New Jersey Gov. Chris Christie.

"If I make a decision to get behind a candidate, I'm gonna get behind a candidate. It's not an endorsement. It's really a movement that this country is looking for. They're looking for a message movement," Sununu said at a Haley event in November.

Haley, a former South Carolina governor and U.N. ambassador in the Trump administration, has seen her poll numbers steadily tick up in recent months, according to 538.

Still, she and other candidates like DeSantis trail Trump by double digits nationally and in various states, despite Trump's legal troubles. (He denies all wrongdoing.)

Haley also recently won the backing of Americans for Prosperity Action, a powerful political advocacy group backed by billionaire Charles Koch.

DeSantis, for his part, has been endorsed by the governor of another early voting state, Iowa's Kim Reynolds; and Trump has a slew of support including from House Speaker Mike Johnson.

A spokesperson for Christie brushed off Sununu's expected endorsement later on Tuesday, after Christie courted Sununu's endorsement for months -- skipping over Iowa to focus heavily on New Hampshire. "This puts us down one vote in New Hampshire," the spokesperson said.

A DeSantis spokesperson said in their own statement that the results of Iowa, not New Hampshire, would be where "the true Trump alternative will emerge."

ABC News' Gabriella Abdul-Hakim, Libby Cathey, Abby Cruz, Hannah Demissie, Nicholas Kerr and Will McDuffie contributed to this report.