Donald Trump romped to a lightning-quick victory Monday in Iowa's caucuses -- the first vote in the US presidential race -- cementing his status as the presumptive Republican standard-bearer to challenge President Joe Biden in November's election.
The former president has led polling for more than a year, but the contest was seen as the clearest insight yet into whether he can convert his advantage into a stunning White House return.
Major US networks took just 30 minutes to project the winner, with Trump opening up an unprecedented 32-point lead over second-placed Ron DeSantis over the following hour or so.
The Florida governor and Trump's other main rival -- former UN ambassador Nikki Haley -- appeared to be locked in a tussle for the runner-up spot at around 20 and 19 per cent respectively.
There had been questions as to whether Trump might have been hamstrung by his legal problems, as he faces civil and criminal trials in multiple jurisdictions in 2024.
But the Iowa victory demonstrated the 77-year-old's success in turning his prosecutions into a rallying cry that has galvanized his followers as he takes his momentum into New Hampshire, the next state to nominate, next Tuesday.
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.
Latcham told AFP that he wanted Trump to "get back to DC," adding that the Republican was "going to help (control) the border" where migrants cross from Mexico -- a key issue in US politics.
Bundled up Iowa residents shuffled into more than 1,600 voting locations, braving sub-zero temperatures in a winter storm that forced candidates to cancel events at the last minute.
Aides of all the hopefuls had fretted over turnout, with some justification, as the number of caucus-goers was estimated shortly after the contest opened to be around 130,000 -- substantially down on the 187,000 who turned out in 2016.
The margin of Trump victory was always the main question of the night, with analysts arguing that a gap above 30 points, or a voting share of more than 50 per cent, would be far above par.
The Trump machine is better organised than when he lost Iowa in 2016, with boots on the ground across the early nominating states.
He has been staging rallies in front of crowds where he has driven home with uncharacteristic discipline his message that immigration and border security are America's biggest issues.
The RealClearPolitics polling average shows Trump leading the Republican field nationally with 61.4 percent support. Haley and DeSantis trail at 12 per cent and 10.7 percent, respectively.
The Iowa result is seen as critical for DeSantis, who shifted significant resources to the state and spent months wooing voters in all 99 counties.
Analysts say anything short of a second-place finish would be disastrous for the hard-line conservative, who is seeking to regain ground lost to Haley.
"I knew Trump would win. I just wanted to help DeSantis maybe get into second," said Julia Talerico, who came with her mother to caucus in Des Moines for the Florida governor.
"I voted Trump first couple of times around. I just feel like we need a change and see some younger blood."
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.
"Iowa is Trump country, and it's heavily Republican and evangelical. New Hampshire will be much more interesting," said veteran political analyst Larry Sabato.
Haley has campaigned on being more electable than Trump, who has sparked outrage -- and comparisons with Adolf Hitler -- for complaining of undocumented migrants "poisoning the blood" of America.
"In every one of those head-to-head polls against Biden, I defeat Biden by double digits," Haley said.
But Trump aides have been clear they want to see off the competition long before the Republican National Convention in July -- and are keen for the party to coalesce around the front-runner before multiple expected court dates.
Much of the campaign has been overshadowed by the legal woes facing the scandal-plagued tycoon, who has sought to use courthouses across the country to dominate TV coverage and to rally support.
The Iowa contest featured some low-polling candidates too, including biotech entrepreneur Vivek Ramaswamy and former Arkansas governor Asa Hutchinson.
Caucuses are also being held by Iowa's Democrats, along with voting by mail until March, with Biden facing two challengers but no serious threat.
