Nikki Haley Endorsed by Gov. Chris Sununu of New Hampshire

Gov. Chris Sununu of New Hampshire endorsed Nikki Haley for the Republican presidential nomination at a campaign event Tuesday evening, saying her poll numbers and ground game “were absolutely unbelievable.”

“We are all in for Nikki Haley,” Mr. Sununu said to loud cheers at a ski area in Manchester.

The endorsement is a significant victory for Ms. Haley, who is trying to establish herself over Gov. Ron DeSantis of Florida as the main alternative to former President Donald J. Trump and has gained ground in New Hampshire polling in the past month.

Mr. Sununu, a Trump critic who is serving his fourth and last two-year term as governor, was re-elected last year by more than 15 percentage points and is popular in the state. He was seen as a top recruit for the Senate last year but declined to run, and he also chose not to run for the Republican presidential nomination himself — saying at the time that he thought he could have more influence as an external voice than as a candidate.

On the 2024 presidential campaign trail, Mr. Sununu stumped with Ms. Haley, Mr. DeSantis and former Gov. Chris Christie, as he weighed which of the three to back. In an interview last month, he said he would talk over his decision with friends and family over the Thanksgiving break. He said he was looking for someone who could beat Mr. Trump and who could connect with voters on “a very retail level.”

“You always want a fighter — you always want someone who is going to put passion and energy behind it,” he said then, adding that as a former governor, Ms. Haley knew how to listen to people and was getting a lot of voters excited.

Given his popularity and his proven ability to win as a Republican in a state that leans Democratic, Mr. Sununu could help sway the moderate Republicans and independents whom Ms. Haley is counting on to give her a strong showing in New Hampshire’s first-in-the-nation primary on Jan. 23.

Undeclared voters, who can participate in the Republican primary, now make up roughly 39 percent of voters in the state, a greater slice of the electorate than either Democrats or Republicans. And with no competitive Democratic presidential primary next year, they are expected to play an even larger role in the Republican contest.

“It is really a big move,” Matthew Bartlett, a former Trump appointee and Republican strategist who is unaligned in the race, said of Mr. Sununu’s backing. “It is really the last chess piece to fall in line before Election Day, and it is not to be underestimated.”

But just how much weight it will carry is an open question in a primary in which nothing — not endorsements, not debates, not 91 felony charges — has changed the basic dynamic: Mr. Trump is the overwhelming favorite, and everybody else is fighting for second place.

The Sununu endorsement was first reported by WMUR earlier on Tuesday.

Mr. DeSantis received two of the biggest endorsements available in Iowa — those of Gov. Kim Reynolds and the evangelical leader Bob Vander Plaats — but has yet to make significant gains on Mr. Trump there. Still, campaign officials for Mr. DeSantis and Mr. Christie downplayed the impact of Mr. Sununu’s backing.

“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,” said Karl Rickett, campaign spokesman for Mr. Christie, who has made New Hampshire his do-or-die state.

Ray Buckley, the chairman of the New Hampshire Democratic Party, criticized both Ms. Haley and Mr. Sununu in a statement. “No matter how much Nikki Haley or Chris Sununu try to spin Granite Staters, the reality is they’re both MAGA extremists who spent years cozying up to Donald Trump,” he said.