Donald Trump swept to 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 Iowa contest was seen as the clearest insight yet into whether he can convert his advantage into a stunning White House return.

In the end, major US networks took just half an hour from the opening of polls to project the winner, with Trump securing almost three quarters of the early vote.

There had been questions as to whether Trump's legal problems -- he faces multiple civil and criminal trials in multiple jurisdictions -- may have dampened his support.

But the Iowa victory would suggest the 77-year-old, who left Washington under a cloud following the 2021 assault on the US Capitol by his supporters, has succeeded in turning those prosecutions into a rallying cry to galvanize his followers.

The opening vote in the primary season, Iowa is considered crucial for winnowing the field and giving those left standing a springboard for the rest of the race.

As he takes his momentum into New Hampshire in eight days, the former reality TV star has a commanding lead that his rivals have been unable to blunt.

Bundled up Iowa residents shuffled into more than 1,600 voting locations across the state, braving sub-zero temperatures in a winter storm that forcing candidates to cancel events at the last minute -- and aides to fret over turnout.  

With voting barely underway as Trump was declared the winner, it was not immediately clear how his closest rivals -- former UN ambassador Nikki Haley and Florida Governor Ron DeSantis were faring.

The margin of the ex-president's victory could be well in excess of the 12-point win his aides said would have made for a good night.

Iowa accounts for less than two percent of the delegates awarded nationwide in the process to pick a party's candidates, so a big night by no means guarantees success in the rest of the nominating season.

But a strong showing is essential for candidates hoping for a boost before New Hampshire, Nevada and South Carolina.

The Trump machine was better organized than when he lost Iowa in 2016, with boots on the ground across the state.

The candidate himself was off the trail in the final week, however, as he made voluntary appearances in some of the many court cases making his tilt at the White House a campaign like no other in history.

The Iowa result is 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, and Haley looked to be narrowly ahead in the first hour of caucusing.

Haley had tried to downplay expectations in Iowa and said she is looking simply for a strong performance ahead of the primary next Tuesday in her preferred state of New Hampshire.

She has repeatedly touted her electability over Trump, pointing to the "chaos" of his criminal cases and reminding Iowans that Republicans have lost the popular vote in seven of the last eight presidential elections.

"I think we've always had a target on our back because we've been the one moving up, everybody else is going down and that's a great thing," Haley told Fox News.

Caucuses -- a quirk of the US election calendar -- are town hall-style meetings involving speeches and debate that only a handful of states stage.

Armies of volunteers have fanned out through Iowa in recent weeks, knocking on doors or manning phone banks, while candidates dominated the air waves with talk show appearances and a barrage of campaign ads.

The caucuses feature some low-polling candidates too, including biotech entrepreneur Vivek Ramaswamy and former Arkansas governor Asa Hutchinson.

Caucuses also are being held by Iowa's Democrats, along with voting by mail until March, with President Joe Biden facing two challengers but no serious threat.