Des Moines, Iowa: Donald Trump secured a resounding victory in the first 2024 Republican presidential contest in Iowa on Monday, asserting his dominance over the party as he seeks a third consecutive nomination and a rematch with Democratic President Joe Biden.

Florida Governor Ron DeSantis and former UN Ambassador Nikki Haley were in a battle for second place, Edison Research projected, as they sought to emerge as the chief alternative to Trump, who served as president from 2017-2021.

Trump was poised to win by an unprecedented margin for an Iowa Republican contest, strengthening his case that his nomination is a foregone conclusion, given his massive lead in national polls despite facing four criminal indictments.

"THANK YOU IOWA, I LOVE YOU ALL!!!" Trump wrote on his social media platform, Truth Social.

With more than 60 per cent of the expected vote tallied, Trump had 50.6 per cent, while DeSantis was at 21.4 per cent and Haley 19.4 per cent, according to Edison. The largest margin of victory for an Iowa Republican caucus was 12.8 percentage points for Bob Dole in 1988.

Trump's margin was insurmountable. To come from behind and win, Haley or DeSantis would have to take the vast majority of the remaining vote, a dramatic and unlikely turnaround.

It was too early to say whether Trump would exceed 50 per cent, a psychological figure that would further weaken his rivals' argument that his march to the nomination can be derailed.

In a statement, Alex Pfeiffer, a spokesperson for the main super political action committee supporting Trump, said, "The people of Iowa sent a clear message tonight: Donald Trump will be the next Republican nominee for President. It's now time to make him the next President of the United States."

Both DeSantis and Haley were aiming for a strong second-place finish to convince donors and supporters that their challenges to Trump remain viable.

DeSantis in particular had wagered his campaign on Iowa, barnstorming all of its 99 counties, and a third-place finish could increase pressure for him to end his bid.

Polls show him far behind Trump and Haley in the more moderate Northeastern state of New Hampshire, where Republicans will choose their nominee eight days from now.

Iowans braved life-threatening temperatures to gather at more than 1,600 schools, community centers and other sites for the state's first-in-the-nation caucus, as the 2024 presidential campaign officially got under way after months of debates, rallies and advertisements.

Caucus-goers appeared broadly supportive of Trump, according to an Edison Research entrance poll.

Only one-third of caucus-goers said Trump would be unfit for president if convicted of a crime. Nearly two-thirds said they did not believe Biden legitimately won the 2020 election, embracing Trump's falsehoods about voter fraud.

"Trump is very narcissistic, he's very cocky, but he's going to get stuff done," said Rita Stone, 53, a Trump backer, who attended a caucus at a West Des Moines high school. Like many other voters, Stone said her leading concern was the US southern border with Mexico, praising Trump's effort to build a wall when he was president.

Trump has aimed to create an air of inevitability around his campaign, skipping all five of the Republican debates thus far and largely eschewing the county-by-county politicking that most candidates do ahead of the Iowa vote.

Life-threatening cold

Unlike a regular election, Iowa's caucus requires voters to gather in person in small groups, where they cast secret ballots after speeches from campaign representatives.

The wind chill in parts of the state had been forecast to reach minus 45 degrees Fahrenheit (minus 43 degrees Celsius) on Monday, according to the National Weather Service.

Edison projected there would be approximately 120,000 votes counted, far short of the record 187,000 cast in the 2016 Republican caucus.

If the cold weather helped depress turnout, Trump's grip on his most loyal supporters may have given him an edge.

DeSantis and Haley had expressed confidence they would exceed expectations in Iowa, though neither predicted victory.

"If you're willing to brave the cold and turn out for me, I'll be fighting for you for the next eight years, and we're going to turn this country around," DeSantis told a crowd earlier in the day in Sergeant Bluff.

At a diner in Des Moines, Haley predicted that other candidates will be forced to drop out in the weeks to come. "This will be a two-person race with me and Donald Trump," she told supporters.

Iowa Democrats did not vote on Monday for their presidential nominees because the party has reshuffled its nominating calendar to put states with more diverse populations ahead of Iowa this year. They will cast their ballots by mail, with the results to be released in March.

Iowa has historically played an outsized role in presidential campaigns due to its early spot on the campaign calendar.

But the winner of Iowa's Republican caucuses did not go on to secure the nomination in the last three competitive contests in 2008, 2012 and 2016.

A political battleground that backed Democrat Barack Obama in 2008 and 2012, the state is now seen as reliably Republican in presidential elections as registered Republicans edge out Democrats.