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

Sununu will formally give 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, according to local ABC affiliate WMUR.

“I look forward to joining Nikki at her town hall this evening – it’s going to be a lot of fun!” Sununu, 49, said in a statement to The Post.

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 announced he would not seek 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’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.

Florida Gov. Ron 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.

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