Republican New Hampshire Gov. Chris Sununu endorsed former South Carolina Gov. Nikki Haley for the party’s presidential nomination Tuesday, giving her a vital boost in the first-in-the-nation primary state.

Sununu formally gave his blessing to Haley at a town hall event in Manchester, the Granite State’s largest city, after spending months evaluating the GOP field for an alternative to former President Donald Trump.

“There was a sweet older woman who has come to a lot of events … she said, ‘So, are you going to finally endorse Nikki Haley for president?’” Sununu, 49, told a packed house at McIntyre Ski Area

“You bet your ass I am,” the governor answered. “Let’s get this thing done. We’re all in on Nikki Haley.”

Haley celebrated receiving Sununu’s seal of approval, telling the town hall attendees that “to go and get endorsed by the Live Free or Die governor is about as rock solid of an endorsement as we could get.”

After grabbing the mic, Sununu took to social media to praise the former ambassador to the United Nations.

“Nikki Haley is the candidate with the momentum to win and get our party back on track to delivering conservative victories across the country,” the governor wrote on X. “She’s answered our questions and built trust. Proud to endorse Nikki Haley for President and hope you’ll join me in this movement!”

Haley, 51, is currently polling second in New Hampshire with an average of 18.7% support, according to RealClearPolitics — exactly 27 percentage points behind the 77-year-old Trump, who Sununu has ruled out backing.

“He can’t win in November of ’24,” Sununu told CNN during a June interview, in which he ruled out seeking the presidency himself.

“The math has shown Donald Trump has no chance of winning in November of ’24. He wouldn’t even win Georgia. If you’re a Republican that can’t win Georgia in November of ’24, you have no shot, and he’s proven that.”

Sununu also slammed Trump in the same month for “cozying up” to disgraced former New York Gov. Andrew Cuomo after the former president argued that the former New York chief executive handled the COVID-19 pandemic better than Florida Gov. Ron DeSantis.

“I literally saw Donald Trump cozying up to Andrew Cuomo,” he told Fox News at the time. “I mean, Andrew Cuomo is the punchline to Republican jokes — even Democrats don’t like Andrew Cuomo. And now Trump and Cuomo are best friends. That is not the head of the Republican Party. That is not a strong conservative message.”

Sununu’s endorsement of Haley represents a massive blow to former New Jersey Gov. Chris Christie, who has essentially staked his campaign’s viability on New Hampshire, where the RCP average has him third.

“We have seen the reports of Governor Sununu’s decision to endorse Nikki Haley. This 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,” Christie’s spokesperson Karl Rickett said in a statement to The Post.

DeSantis, meanwhile, has honed in on the first caucus state of Iowa, sending part of his staff to Des Moines and hitting all 99 counties ahead of the Jan. 15 contest.

The Sunshine State governor also received the endorsement of Iowa Gov. Kim Reynolds, while another early state governor, South Carolina’s Henry McMaster, has backed Trump.

DeSantis responded to Sununu’s endorsement by saying he would not “trash” the governor, an apparent reference to Trump going after Reynolds.

“Unlike some people running, if someone doesn’t endorse me, I’m not gonna go trash them,” DeSantis said at a CNN town hall Tuesday night.

“Chris is a good guy. He’s done a good job, and I’m gonna continue to say he’s good — he’s a really good campaigner, I mean, excellent campaigner. So, in that sense, I think it’s a benefit for her but here’s the problem, even a campaigner as good as Chris is not going to be able to paper over Nikki being an establishment candidate,” DeSantis added. 

Tricia McLaughlin, spokesperson for presidential candidate and biotech mogul Vivek Ramaswamy, reacted by saying Haley “deserves” the endorsement.

Sununu and Haley “are cut from the same establishment cloth. It’s the perfect endorsement,” McLaughlin told The Post.

Sununu is consistently ranked as one of the most popular governors in the country, according to polling data. 

The son of former White House Chief of Staff John Sununu won his 2020 re-election contest by over 30 percentage points. At the same time, Trump lost New Hampshire to President Biden by 7.4 percentage points. 

In 2022, Sununu easily defeated his primary opponent, receiving 78% of the vote, and cruised to another term by 15.5 percentage points.

However, he has opted not to seek a fifth two-year term in 2024, which would have been unprecedented in the state’s modern history.