Republican hopeful Nikki Haley has received a welcome boost in a crucial state in the form of an endorsement from a popular governor.

Ms Haley on Tuesday received the enthusiastic backing of sitting New Hampshire governor, Republican Chris Sununu.

“This is an opportunity for New Hampshire to lead this country, for New Hampshire to say we’re not looking in the rearview mirror anymore,” Mr Sununu said at the event in Manchester.

Ms Haley has worked hard to position herself in second place in New Hampshire, still far behind Donald Trump, but several points ahead of Chris Christie and Ron DeSantis.

If she manages to get a decent result in Iowa, which is the first state to vote, she would hope for momentum in New Hampshire to then carry her onto states such as South Carolina.

Joining Mr Sununu on stage, Ms Haley called it “about as rock solid as an endorsement as we could hope for”.

She added: “It’s a great night in New Hampshire, I mean it doesn’t get any better than this.”

In a message for Mr Trump, Mr Sununu said: “Thank you for your service, Mr. President, we’re moving on. This is New Hampshire, and we go forward.”

One unanswerable question at this stage is what difference such an endorsement might make, given Mr Trump’s relentless grip on the party.

A recent poll by the Des Moines Register put him at 51 points in Iowa, where Mr DeSantis recently received the endorsement of Iowa governor Kim Reynolds, but barely saw his numbers move.

An average of polls collated by Real Clear Politics puts Mr Trump on 50 points in Iowa, with Mr DeSantis on 20 and Ms Haley on 16. In New Hampshire, his lead is anywhere as much as 27 points.

As an early state governor, Mr Sununu’s endorsement holds disproportionate weight, and he has the backing of the majority of Republicans in New Hampshire.

At an event in New Hampshire last month, Ms Haley joked with Mr Sununu: “Are you ready to endorse me yet?”

To the surprise of some observers, he responded: “Getting closer every day.”

The Republican primaries begin on January 15 in Iowa, where voters will caucus for their preferred candidate. Mr Trump remains ahead there, but Ms Haley and Mr DeSantis will compete for second place in an attempt to become the favoured alternative candidate.

Responding to reports of Ms Haley’s recent success in the polls, Mr Trump wrote on Sunday night: “Where’s the Nikki Surge? I hear about it from the Fake News Media, but don’t see it in the Polls, or on the Ground.”

Using his nickname for Mr DeSantis, he added: “In any event, I hope she and DeSanctimonious are doing well, and continue the same “Surge” as they’ve had for the past eight weeks!”

Mr Sununu has previously said that if he chose to endorse a candidate, he would want to “help them put together a campaign, a ground game,” noting that it is “the emphasis, the energy you put behind it, the campaign, the messaging” that “really matters”.