MANCHESTER, N.H. - Saying "let’s not miss this opportunity. The entire country is watching," Republican Gov. Chris Sununu of New Hampshire took sides in the GOP presidential nomination race, as he endorsed former ambassador to the United Nations and former South Carolina Gov. Nikki Haley.

The endorsement of Haley by the popular governor of the state that holds the first primary and second overall contest in the Republican presidential nominating calendar came as the two teamed up Tuesday evening at a Haley campaign event at a ski lodge in the state's largest city.

"There was a sweet older woman who has come to a lot of events and I saw her coming in here and she said, ‘So are you going to finally endorse Nikki Haley for president?’ You bet your a-- I am. We’re all in for Nikki Haley," Sununu said as he formally backed Haley.

Moments later, Sununu praised Haley as "someone I could not be more proud of. Someone who looks people in the eye, answers their questions and most importantly, has taken the time to earn the trust of the citizens and voters in this state. The next President of the United States – Nikki Haley."

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Sununu endorses Haley

Republican Gov. Chris Sununu of New Hampshire (right) endorses former ambassador and former South Carolina Gov. Nikki Haley, at a campaign event in Manchester N.H. on Dec. 12, 2023  (Fox News - Paul Steinhauser)

A beaming Haley, speaking after Sununu, said "it’s a great night in New Hampshire. I mean it doesn’t get any better than this. To go and get endorsed by the live free or die governor is about as rock-solid endorsement as we could hope for."

Haley, in a rare question and answer session with reporters following her town hall, said the Sununu endorsement is "huge for us. It really is. I and everybody else has been courting Chris for months. It’s no secret. You all saw it…. we’ve got momentum on our side but this just gave it a speed bump. So we’re excited about going and going fast."

The endorsement by Sununu, who's won election and re-election to four two-year terms as governor, could potentially sway some Republican voters, as well as independents and moderates who often play an influential role in New Hampshire's crucial presidential primary.

Sununu, who for nearly three years has been a vocal critic of former President Donald Trump – the commanding front-runner for the 2024 GOP nomination as he makes his third straight White House run - flirted with his own presidential bid before announcing in early June that he wouldn't seek the White House.

Since then, he said he would eventually endorse in the Republican nomination race and teamed up repeatedly with many of the GOP contenders as they've campaigned in New Hampshire. 

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In recent weeks, he highlighted that he had narrowed his endorsement choice down to three candidates – Haley, former New Jersey Gov. Chris Christie, and Florida Gov. Ron DeSantis.

And Sununu campaigned with all three candidates on the trail in New Hampshire just before Thanksgiving.

Sununu told reporters following Tuesday's campaign event that "this is a race between two people – Nikki Haley and Donald Trump. That’s it."

And Sununu highlighted minutes earlier in a Fox News Digital interview that "clearly there is a movement here behind Nikki Haley. Cleary there is a connection she’s making in all of the three early states…. There’s opportunity for her across the board and it’s with the Republican base, it’s with these undeclared, it’s independents."

Sununu said that his endorsement will be much more than a one day announcement. 

He pledged that "for the next six weeks [leading up to New Hampshire's Jan. 23 primary] we’re going to go very hard on this campaign….whether it’s her and I on the campaign trail. A little bit of her. A little bit of me. We’re going to just take the state by storm and make sure everyone has heard not just the message but the opportunity."

Sununu is scheduled to join Haley at three town halls across New Hampshire on Wednesday and Thursday.

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Christie, who is once again spending most of this time and resources on New Hampshire as he makes his second White House run, returns to the campaign trail in the Granite State with two events on Wednesday. 

Christie and Sununu team up on the campaign trail in New Hampshire

Former New Jersey Gov. Chris Christie (left), a 2024 Republican presidential candidate, teams up with GOP Gov. Chris Sununu of New Hampshire at a town hall in Merrimack, N.H., on Nov, 9, 2023. (Fox News - Deirdre Heavey)

As he worked to land Sununu's endorsement, Christie spotlighted that when it comes to Trump, he and the New Hampshire governor were on the same page.

"Who does he want standing across from Donald Trump when this gets down to a one-on-one? Who does he think can take him on in a direct way? Who’s been saying the same things as Chris Sununu has been saying for the last couple of years about Donald Trump, trying to move the party in a new direction? And I think I’m the person who has the clearest, strongest voice on that," Christie emphasized in a Fox News Digital interview a couple of weeks ago.

His campaign put out a statement on Tuesday afternoon noting that Sununu's endorsement of Haley "puts us down one vote in New Hampshire and when Governor Christie is back in Londonderry tomorrow, he’ll continue to tell the unvarnished truth about Donald Trump and earn that one missing vote and thousands more."

Asked in his Fox News interview if Christie should drop out of the race, Sununu said "I’m behind Nikki Haley. I think they should all get out frankly, including former President Trump. I think everyone should kind of clear the way."

But he acknowledged "they’re going to keep campaigning."

Haley, asked by Fox News Digital if Christie should call it quits, said "Chris is my friend and I will never tell anyone to get out of the race. It’s a personal issue to get in. It’s a personal decision to get out. That’s Chris’ decision to make."

DeSantis, who is spending most of his time in Iowa, is expected back in New Hampshire on Friday.

"What happens in New Hampshire will be significantly impacted by the outcome in Iowa, where the true Trump alternative will emerge. And when Ron DeSantis comes out in that position, he will be joined by over 60 New Hampshire state legislators who stand ready to take the fight to the establishment and their candidates of yesteryear to return power to grassroots conservatives," DeSantis campaign spokesman Andrew Romeo argued in a statement.

And in a town hall in Iowa hosted by CNN, DeSantis on Tuesday evening argued that "even a campaigner as good as Chris Sununu is not going to be able to paper over Nikki Haley being an establishment candidate."

Sununu endorses Haley

Republican presidential candidate and former U.S. Ambassador to the United Nations Nikki Haley is endorsed by New Hampshire Governor Chris Sununu at a campaign town hall in Manchester, New Hampshire, on December 12, 2023.   REUTERS/Brian Snyder (REUTERS/Brian Snyder)

Haley, who has enjoyed momentum in the polls in recent months, thanks in part to well-received performances in the first three GOP presidential primary debates, leapfrogged DeSantis for second place in New Hampshire and her home state, which holds the first southern contest. Christie stands in third place in most of the latest surveys in New Hampshire.

Haley also aims to make a fight of it in Iowa – the state whose Jan. 15 caucuses lead off the GOP nominating calendar. The latest polls suggest she is close to pulling even with DeSantis for a distant second place behind Trump.

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Sununu's endorsement was one Haley had long coveted.

Early this summer, at the New Hampshire GOP's annual cookout, Haley was introduced by Sununu. After exchanging a hug, Haley kicked off her comments to the crowd by saying, "You’ve got a great governor."

With a joke that elicited plenty of laughter, she said, "Governor, I very much worry about your health. What I’m thinking is, I don’t want you to over-stress. I don’t want you to get out there and do too much. So I think what’s best is, go ahead and endorse me now."

Haley was kidding, but in the ensuing months she's jokingly asked Sununu about an endorsement a handful of times.

"Are you ready to endorse me yet?" Haley asked Sununu as they teamed up at a campaign stop last month.

"Getting closer every day," Sununu replied as he smiled.

Sununu's backing of Haley follows by a month Republican Gov. Kim Reynolds of Iowa's endorsement of DeSantis. Since then, Reynolds has joined DeSantis at multiple stops on the Hawkeye State campaign trail. 

Iowa Governor Kim Reynolds Endorses GOP Candidate Ron DeSantis For President

Gov. Ron DeSantis speaks with Iowa Gov. Kim Reynolds during a campaign rally on Nov. 6, 2023, in Des Moines. (Scott Olson/Getty Images)

The big question going forward is whether Sununu's endorsement will impact a race dominated by Trump.

The governor has tempered expectations that his endorsement might move the needle in the Granite State, telling Fox News last month that "I’m never a big believer that endorsements matter as much as the press think they do."

On Tuesday, after backing Haley, the governor told Fox News that "it’s not just about my endorsement, but I think some of the news and media that the endorsement kicks off gets people excited to look in this direction."

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Longtime New Hampshire-based Republican strategist Jim Merrilll, a veteran of numerous presidential campaigns, told Fox News the endorsement was "a big deal."

"There's no endorsement in New Hampshire you'd rather have. And it's clear he's going to use it early and often to support Haley."

And Neil Levesque, the executive director of the New Hampshire Institute of Politics at Saint Anselm College, told Fox News "it is always questionable whether endorsements matter but in this case, this is a game-changing endorsement."

"This is why," Levesque continued. "Sununu is one of the most talented communicators in politics. You combine that with his credibility and popularity in a state like New Hampshire and the fact that he’s going to basically go on the road and sell this to New Hampshire voters and make a persuasive argument for Nikki Haley, I think is going to move the needle."

But longtime New Hampshire-based Republican consultant Mike Dennehy told Fox News "I think times have changed from the 1990s when endorsements by a governor made a significant difference."

"I expect Sununu will open some doors and he will make for a very good surrogate, but in the end I don't think it will make more than a one percent difference in this race," Dennehy, a veteran of numerous GOP presidential campaigns, said.

Sununu's backing of Haley comes a couple of weeks after she landed the endorsement of Americans for Prosperity Action, the political wing of the influential and deep-pocketed fiscally conservative network founded by the billionaire Koch Brothers. AFP Action has pledged to spend tens of millions of dollars and mobilize its formidable grassroots operation to boost Haley and help push the Republican Party past Trump.

Trump at rally

Former President Donald Trump leaves the stage at a campaign rally Saturday, Nov. 11, 2023, in Claremont, New Hampshire. (AP Photo/Reba Saldanha)

The Trump campaign on Tuesday took aim at Sununu.

"Sununu’s endorsement means nothing and does nothing for any candidate in this race," Trump campaign spokesperson Steven Cheung charged in a statement to Fox News. "The only endorsement in politics that matters is President Trump’s endorsement. Nothing will stop him from securing the nomination and beating Crooked Joe Biden and retaking the White House."

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Trump continues to hold a very formidable and very large double-digit lead over Haley, DeSantis and the rest of the remaining field of rivals for the nomination in the latest polls in New Hampshire, Iowa, South Carolina, and in national surveys.

But Sununu said last month that Trump's "got a floor, but he’s also got a ceiling." 

"And when you look at the fact that well over 50% of the Republican core-based voter wants somebody else, the fact that in New Hampshire you can have independents that come out – I believe in record numbers – most of which won’t vote for yesterday’s news in terms of Donald Trump," Sununu argued.

And on Tuesday he took a shot at both Trump and President Biden, charging that "we have a president that’s more concerned about his nap time, and a former president that’s worried about his jail time."

Get the latest updates from the 2024 campaign trail, exclusive interviews and more at our Fox News Digital election hub.