The two have competed aggressively in recent weeks to emerge as the clear alternative to the former president, who has alienated many Americans and could end up being a convicted felon by year’s end.

“For me, it’s really about character,” said Darla De Haan, a psychotherapist from Pella, explaining why she is leaning to Haley. “I want to see people who have integrity, who keep their word. … I kind of get a sense when you’re around someone, if they’re going to do what they say.”

Hans Rudin, a 49-year-old community college adviser from Council Bluffs, said he has supported Trump in the past but is now aligned with DeSantis.

Nonetheless, polls suggest Trump entered Monday with a massive lead in Iowa as Haley and DeSantis duel for a distant second because of voters such as Kathy DeAngelo.

“Trump is a Christian. He’s trustworthy. He believes in America. And he believes in freedom,” said DeAngelo, a 71-year-old a retired hospital administrative employee. “He’s the only one.”

Entrepreneur Vivek Ramaswamy and former Arkansas Governor Asa Hutchinson are also on the ballot in Iowa, as is former New Jersey Governor Chris Christie, who suspended his campaign last week.

With the coldest temperatures in caucus history and dangerous travel conditions in virtually every corner of the state, the campaigns are bracing for a low-turnout contest that will test the strength of their support and their organisational muscle. The final result will serve as a powerful signal for the rest of the nomination fight to determine who will face Democratic President Joe Biden in the November general election.

After Iowa, the Republican primary shifts to New Hampshire, Nevada and South Carolina over the coming weeks before moving into the rest of the country this spring. The ultimate nominee will not be confirmed until the party’s national convention in July, but with big wins in the opening contests, Trump will be difficult to stop.

Trump’s political strength heading into the Iowa caucuses, which come 426 days after he launched his 2024 campaign, tells a remarkable story of a Republican Party unwilling or unable to move on from him. He lost to Biden in 2020 after fuelling near-constant chaos while in the White House, culminating with his supporters carrying out a deadly attack on the US Capitol.

In total, Trump faces 91 felony charges across four criminal cases, including two indictments for his efforts to overturn the election and a third indictment for keeping classified documents in his Florida home.

In recent weeks, Trump has increasingly echoed authoritarian leaders and framed his campaign as one of retribution. He has spoken openly about using the power of government to pursue his political enemies.

Trump has repeatedly harnessed rhetoric once used by Adolf Hitler to argue that immigrants entering the US illegally are “poisoning the blood of our country”. And he recently shared a word cloud last week to his social media account highlighting “revenge”, “power” and “dictatorship”.

His campaign trotted out some high-profile conservative populists to greet Iowans in Fort Dodge.

“I want to tell you how much the world is depending on Iowa tonight,” said Kari Lake, who is running for the Senate in Arizona. She was joined by congressman Jim Jordan of Ohio, congressman Matt Gaetz of Florida, and congresswoman Marjorie Taylor Greene of Georgia.

“I truly believe that God has placed each and every one of us here at this moment, for this moment to save this country,” said Lake, who grew up in Iowa.

The final Des Moines Register/NBC News poll before the caucuses found Trump maintaining a formidable lead, supported by nearly half of likely attendees, compared with 20 per cent for Haley and 16 per cent for DeSantis, who are in a close battle for second. Trump is also viewed more favourably than the other top contenders by likely caucus attendees, at 69 per cent compared with 58 per cent for DeSantis and 48 per cent for Haley.

Trump used his Truth Social site to knock Haley. Her campaign answered that Trump’s diatribes, some of them contradictory, prove he considers her a threat.

Haley began the day in central Iowa by encouraging some of her caucus precinct captains to “speak from the heart” as they make their statements on her behalf before votes are cast.

Trump predicted he would set a modern-day Republican caucus record with a margin-of-victory exceeding the nearly 13 percentage point victory that Bob Dole earned in 1988. Still, Trump played down expectations that he would draw a majority of the total vote.

The temperature in parts of Iowa on Monday could dip as low as minus 14 degrees Fahrenheit (minus 26 degrees Celsius) while snow drifts from Friday’s blizzard still make travel hazardous across the state.

Forecasters warned that “dangerously cold wind chills” as low as minus 45 degrees Fahrenheit (minus 42 degrees Celsius) were possible until noon on Tuesday. The conditions, according to the National Weather Service, could lead to “frost bite and hypothermia in a matter of minutes if not properly dressed for the conditions”.