Manchester, New Hampshire - Donald Trump won the key New Hampshire primary Tuesday, moving him ever closer to locking in the Republican presidential nomination and securing an extraordinary White House rematch with Joe Biden.

With vote counting ongoing, it was unclear if Trump had secured the knockout victory to put his sole remaining challenger, Nikki Haley, out of the contest.

In a speech following the vote, the former UN ambassador during Trump's frequently chaotic presidency said the race was "far from over" and told supporters that Democrats "want" to run against her former boss.

"They know Trump is the only Republican in the country who Joe Biden can defeat," Haley (52) warned.

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With strong turnout in the northeastern state, Haley had hoped for a major upset. But broadcasters quickly projected her defeat as the first tallies came in. Trump was already the runaway leader in national Republican polling despite two impeachments as president and four criminal trials hanging over him since leaving office.

While Haley repeatedly questioned the 77-year-old's mental fitness and warned another Trump presidency would bring "chaos," polls indicate her efforts in New Hampshire created little more than a speed bump.

"I think it's a two-person race now between Trump and Biden," Keith Nahigian, a veteran of six presidential campaigns and former member of Trump's transition team, told AFP.

New Hampshire was markedly more Haley-friendly than states she will subsequently face, should she stay in the race, and continuing into February will be a tough sell without a win or at least a narrow loss.

Her next must-win stop will be her home state South Carolina.

Trump won a crushing victory in the first Republican contest in Iowa last week, with Haley a distant third.

What was once a crowded field of 14 candidates narrowed to a one-on-one match-up on Sunday after Florida Governor Ron DeSantis dropped out following his second-place Iowa finish.

No Republican has ever won both opening contests and not ultimately secured the party's nomination.

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Trump did little actual campaigning in New Hampshire. However, his message — a mixture of personal grievance and right-wing culture war firing his base — has delivered seemingly insurmountable polling leads.

One of Trump's complaints has been his false claim that Democrats are allowed to vote in the Republican contest in New Hampshire. However, independents are allowed to vote, and Haley had hoped they would revolt against Trump, seeing her as the moderate alternative.

She spent the week hammering the message, backed by polling that most Americans do not want to see a Trump-Biden rematch.

"Nikki Haley's supporters will surely feel that Tuesday night in New Hampshire was a reasonably good night. But once the relative shine of the Granite State result wears off... all but the most ardent Haley supporters will be looking through a glass darkly," said Aron Solomon, a political analyst for legal marketing agency Amplify.

New Hampshire Democrats also voted for their standard-bearer Tuesday, defying a national party order to hold the primary later. Biden did not file candidate paperwork after a row between national and state party officials.

Broadcasters projected that Biden won as a "write-in" candidate, even though it was not an official Democratic Party primary following the row.

After the New Hampshire result came through, the Biden campaign said Trump had "all but locked up" the nomination.

"The election-denying, anti-freedom MAGA movement has completed its takeover of the Republican Party," the campaign said in a statement.