Des Moines:Voters venture into sub-zero temperatures on Monday to kick off the US Republican presidential nomination race with the Iowa caucuses, the first major test of whether front-runner Donald Trump can beat out rivals Nikki Haley and Ron DeSantis.

With a commanding lead in polls, the ex-president is expected to win the Midwestern state's first-in-the-nation vote handily as he bids to be the Republican standard-bearer against President Joe Biden in November.

But Iowans may have to contend with the coldest conditions in the modern era of presidential election campaigns, with blizzards and a potential wind chill in some areas of -36 degrees Fahrenheit (-38 degrees Celsius) or even colder.

Trump and his leading rivals Haley and DeSantis were forced to cancel appearances in the home stretch of campaigning as the threat to Monday's turnout added intrigue to a campaign season that is already something of an unknown quantity.

"One day from now we're going to win," Trump said at a campaign event in Indianola, just south of capital Des Moines, coming on the heels of him having to scrap three weekend rallies.

"Dress warmly tomorrow," he added. "Brave the weather, go out, and save America."

Despite his apparent strength, the former president has been indicted four times since he was last a candidate and is preparing for the potential collapse of his business empire in his native New York as a result of a civil fraud trial.

"If DeSantis's massive ground effort, coupled with a recent Haley surge, can drag Trump under 50 percent by several points, that will be the first meaningful sign that Trump can be defeated," said political analyst Alex Avetoom, who worked on Republican John McCain's 2008 presidential campaign.

"However, this paradigm-shifting reality -- that Trump could be defeated -- happens if, and only if, the rest of the field consolidates behind one anti-Trump candidate."

Poor predictor
For all the talk of miracle bounces, the Iowa race is hardly competitive: A new NBC News/Des Moines Register/Mediacom poll has Trump at 48 percent among likely caucus-goers, with Haley surging into second place but still only at 20 percent.

"I'm voting for Trump again," 37-year-old trucker Jeff Nikolas told AFP, adding that "he may be bullheaded, but he can actually get stuff done."

The poll was more bad news for Florida Governor DeSantis, who scored just 16 percent and has seen his claim to be heir apparent to the post-Trump Republican Party eclipsed by Haley.

But DeSantis insisted Sunday that his "very motivated" backers would turn out in sufficient numbers -- despite the frigid conditions -- to keep him relevant in a vote open only to registered Republicans.

In 2016 only 186,000 Iowans took part in the caucus, he told ABC, and "now, with this weather, it could be significantly less," making turnout paramount. He urged his supporters: "Bring in friends and family, man, that's going to pack a punch."

Haley, a former South Carolina governor, is looking to outperform expectations to cement her claim to be Trump's top challenger going into her preferred state of New Hampshire the following week.

"You don't defeat Democrat chaos with Republican chaos," she told Fox News Sunday. "We have got to put these names of Biden and Trump in the past and look forward with new solutions."

Iowa is a notoriously poor predictor of the eventual nominee but it is considered crucial for winnowing the field and as a springboard to the next few battlegrounds, which include Haley's home state.

Stung by defeat in 2016 after skipping much of Iowa's campaign trail, Trump has built up an impressive network of "precinct captains" to corral votes this time around -- but he has been as notable for courtroom appearances as campaign events.

In a state that likes to meet its candidates face-to-face, DeSantis has been at pains to highlight his own ground game, which has taken him to all 99 counties.

Eye candy
The Iraq veteran and conservative hard-liner will be under heavy pressure to drop out, however, if he finishes third -- although Avetoom cautioned against counting him out.

"Poll respondents are not necessarily Iowa caucus-goers, and the DeSantis precinct operations... are run by first-in-class operatives," he said.

Edward Segal, a former press secretary for Democratic and Republican lawmakers, echoed the benefits of a strong ground game, pointing to at least nine presidents who toured Iowa by train during campaigns.

"Whistle-stop campaign trains can still serve as eye candy to help attract the attention of voters and the media," the analyst told AFP.

The Republican primary also features a number of low-polling candidates, including biotech entrepreneur Vivek Ramaswamy, who has promised a third-place finish in Iowa but didn't qualify for the final televised debate.

Iowa's Democrats will also attend caucuses -- meetings at which local members of a political party gather to register their candidate preferences -- on Monday, but will vote by mail from January until March.

Biden is expected to comfortably defeat self-help author Marianne Williamson and Minnesota congressman Dean Phillips.