Donald Trump has thanked “the great people of Iowa” after he muscled past his rivals to sweep to a dominant victory in the first 2024 Republican presidential contest.

Florida governor Ron DeSantis finished well behind in second place, Edison Research projected, edging out former UN Ambassador Nikki Haley as they battle to emerge as the chief alternative to Trump.

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 even though he faces four criminal indictments.

Trump’s thumping victory would bolster his argument that he is the only Republican candidate capable of taking on Democratic President Joe Biden, despite facing four criminal cases that could go to trial before the November general election.

“Most importantly we want to thank the great people of Iowa,” he said from his campaign headquarters in Des Moines, hours after US media began projecting the victory.

“I really think this is time now for everybody, our country, to come together … Whether it’s Republican or Democrat or liberal or conservative, it would be so nice if we could come together and straighten out the world and straighten out the problems.”

The win again asserts Trump’s dominance over the Republican party as he seeks a third consecutive nomination. He also had a dig for his rivals.

“I want to congratulate Ron and Nikki for having a good time together. We’re all having a good time together. I think they both actually did very well. We don’t know what the outcome of second place is,” Trump said, to laughter from his supporters.

“I also want to congratulate Vivek [Ramaswamy], because he did a hell of a job. He came from zero, and he’s got probably 8 per cent. That’s an amazing job.”

Tweet from @VivekGRamaswamy

With nearly 90 per cent of the expected vote tallied, Trump had 50.9 per cent, while DeSantis was at 21.4 per cent and Haley 19.0 per cent, according to Edison.

The largest margin of victory for an Iowa Republican caucus had been 12.8 percentage points for Bob Dole in 1988.

Entrepreneur Vivek Ramaswamy ended his long-shot presidential bid after earning just under 8 per cent of the vote on Monday and endorsed Trump in a speech to supporters.

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.

Both DeSantis and Haley had been 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 pouring resources into the state.

The result on Monday will undoubtedly keep both DeSantis and Haley in the race, ensuring Trump’s opposition remains fractured as the campaign moves to other states.

Iowans braved life-threatening temperatures of -43 degrees to gather at more than 1600 schools, community centres 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.

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.

Tweet from @JoeBiden

Biden’s campaign was quick to begin fundraising off Trump’s projected Iowa caucus win, warning of “vile attacks, endless lies, and massive spending” if he secured the Republican nomination for president.

“The Iowa results are in, and it’s clear: Donald Trump is the official frontrunner for the 2024 Republican presidential nomination,” an email to supporters late on Monday said.

“We need to work even harder now.”

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.

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.

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.

-with AAP