Donald Trump stormed towards New Hampshire on Tuesday, knowing that a repeat of his runaway win in Iowa would all but seal the Republican nomination to set up his rematch with President Joe Biden in November.

The scandal-plagued former president romped home in the first contest of the drawn-out US presidential race, scoring 51 percent of Republican voters to trounce rivals Ron DeSantis and Nikki Haley for the biggest Iowa caucuses victory in modern history.

The three Republican contenders will meet again for the next contest in New Hampshire next Tuesday and Trump, 77, will have a chance effectively to deliver a killer blow.

"I really think this is time now for everybody, the country, to come together," Trump told a victory rally in Iowa in an unusually conciliatory tone toward his would-be rivals.

But the tycoon and former reality TV star quickly pivoted to his usual harsh rhetoric, promising to shut the Mexican border to stop an "invasion" of migrants and pledging to drill for oil if reelected.

It's an abrasive message that has divided the country and promises a bitter year's campaigning ahead as Trump seeks what he says will be vengeance against Biden. But it's also a message that has delivered Trump a powerful base of support.

"Trump is demonstrating impressive strength among blue-collar, working-class, and rural voters. His victory was not a surprise," Dennis J. Goldford, a political science professor at Drake University in Des Moines, told AFP.

- 'Night for victory' -

Trump now heads to a campaign event in New Hampshire on Tuesday night -- with a stop on the way to appear in court in New York, where he is on trial for defamation in the wake of a jury finding he was liable for a 1990s sexual assault against writer E. Jean Carroll.

The twice-impeached Trump has so far succeeded in turning the civil case -- and the 91 criminal indictments he faces on charges including subverting the 2020 elections -- into a rallying cry claiming that he is the victim of a shadowy "deep state."

Trump has easily led Republican polling for more than a year and the contest in a frozen, blizzard-battered Iowa offered the clearest insight yet into his ability to convert that advantage into a stunning White House comeback campaign.

Major US networks took just half an hour to call the race, with Trump opening an unprecedented 30-point gap over Florida Governor DeSantis, who had pinned his hopes on Iowa. Former UN ambassador Haley took third place with 19 percent.

Heart doctor Allan Latcham, 62, who voted before heading to Trump's election party in Des Moines, hailed a "night for victory," although he admitted he was surprised by the speed of the result call.

- 'Madness' -

DeSantis is also heading there after a quick stop in South Carolina, which holds its primary on February 24, seeking to deal an early blow to Haley who was formerly governor there.

A strong showing in Iowa was seen as essential for DeSantis, who wooed voters in all 99 counties, and was hoping to run Trump close.

DeSantis confirmed that he would stay in the race to "reverse the madness that we've seen in this country" but the governor is considered weak in New Hampshire and many analysts were declaring his campaign all but dead.

Haley, the only woman in the Republican contest, was looking to outperform expectations in Iowa and ride into a one-on-one match-up with Trump in her preferred battleground of New Hampshire.

Despite her third-place finish, Haley dismissed DeSantis's chances and she vowed to avert the "nightmare" of a Trump-Biden rematch by winning in New Hampshire.

"Trump and Biden are both about 80 years old," she said.

The Iowa contest featured some low-polling candidates too, including biotech entrepreneur Vivek Ramaswamy, who dropped out and endorsed Trump after finishing in single digits.

The election is being closely watched around the globe, with key US allies Ukraine and Israel embroiled in wars and many countries anxious about a return of Trump's "America First" policies.

Speaking against a backdrop of US flags, Trump told his Des Moines rally on Monday that "it would be so nice if we could come together and straighten out the world."