Manchester, New Hampshire - Donald Trump aims to sew up the Republican presidential nomination after the contest narrowed to a two-horse race with Florida Governor Ron DeSantis' withdrawal less than 24 hours before the New Hampshire primary on Tuesday.

Short of a defeat in the Granite State, or a close second-place for his remaining rival Nikki Haley, the timeline to Trump's nomination as the candidate to face President Joe Biden in November could be greatly accelerated.

The ex-president won a crushing victory over DeSantis in Iowa last week, with Haley trailing in third, and no candidate has ever failed to claim the Republican crown after taking the two opening states.

That makes New Hampshire make-or-break for Haley, Trump's one-time UN ambassador, who trails her former boss in polling for what is seen as her strongest state.

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Trump (77) has ramped up attacks on Haley over the last week, calling her "not smart enough" and claiming that she had not earned the respect of voters.

He assailed her again Sunday and praised DeSantis as a "very capable person" as he accepted the governor's endorsement in front of cheering supporters.

"Without the endorsement I think we would have got all of those votes," he said to raucous applause at his campaign headquarters in Manchester.

"Because we have very similar policies – strong borders, great education, low taxes, very, very few regulations, as few as possible – things that (Haley) really doesn't talk about, because she's a globalist."

Haley had earlier taken aim at the front-runner's mental acuity after he confused her with veteran Democrat Nancy Pelosi during a rally.

"He's just not at the same level he was at 2016. I think we're seeing some of that decline. But more than that, what I'll say is focus on the fact that no matter what it is, chaos follows him," she told CBS.

With DeSantis out of the picture, Haley is seeking to rely on New Hampshire's high proportion of Independents – who are allowed to vote in either party's primary and typically break for more moderate candidates – to mount what some analysts have described as her "last stand."

But it will be an uphill climb as she is 15 points behind Trump in the RealClearPolitics and FiveThirtyEight polling averages for the contest, and a recent surge in momentum appears to have stalled.

If Haley over-performs on Tuesday, however, that could see her regain the elusive buzz as a genuine threat to the ex-president heading into her home state of South Carolina in late February.

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"I think it would be great for us to have Nikki Haley as president," Madison Gillis, an 18-year-old first-time voter and a Republican, told AFP at a diner in Manchester.

"I think she's amazing. I love what she stands for. I think she has a shot here in New Hampshire."

New Hampshire is a small prize in the grand scheme, allocating just 22 of the 2,429 delegates who nominate at the Republican convention in Milwaukee come July.

But it is a more reliable bellwether for nationwide electoral success than more conservative states, and is seen as setting the tone for the coming races.

So-called "Super Tuesday" voting on March 5, when 874 delegates are on the table, can get a candidate three-quarters of the way to the total required for the nomination.

Aides expect Trump to be in a position to close out the race a week later and want it in the bag by April at the outside – almost certainly before any of his various criminal trials begin.

Democrats have their own primary in New Hampshire on the same day as Republicans, but Biden is not on the ballot after local officials clashed with the national party over scheduling.

Many campaigners say they will vote for Biden anyway, writing his name on the ballot in hopes he can still beat Minnesota Congressman Dean Phillips and progressive challenger Marianne Williamson.

The outcome will not affect the nomination process though, as the Democratic National Committee has declared the New Hampshire primary illegitimate, and the president is expected to win the nomination at a canter.