Donald Trump is projected to win Iowa’s caucuses, the first of the state-by-state contests where Republican voters will pick their White House candidate.
The race for second place, between Nikki Haley and Ron DeSantis, was still up in the air.
Mr Trump won strong support from white evangelicals and very conservative voters, say entrance poll data.
The eventual Republican nominee will challenge the Democrat pick, probably President Joe Biden, in November.
Registered Republicans braved sub-zero temperatures to gather on Monday evening across Iowa to cast ballots at churches, schools, gyms and community centres.
At a high school in Davenport, in the eastern part of the state, Trump supporter Brian Romer gave an impassioned speech to his fellow Republicans in favour of the former president.
“The things that happen in this country are things that happen in communist countries,” he said, referring to Mr Trump’s criminal indictments. “We don’t live in a free country anymore.”
According to the entrance poll data, Mr Trump won men, women, older voters and younger voters, improving on his 2016 performance with all of these groups.
Most Iowa caucus-goers largely dismissed his ongoing legal woes, saying he would still be fit for the presidency even if he were convicted of a crime.
The issue of immigration helped boost Mr Trump: he overwhelmingly won those who picked this as their top issue.
Most Republican voters in Iowa said they favoured a nationwide ban on all or most abortions, and most of those voters also went for Mr Trump.
A pro-Trump political action committee called on his rivals to pull out to avoid a protracted battle and save resources for the general election in November.
“Every dollar spent by President Trump’s primary losers is a dollar that could be fighting Joe Biden,” Alex Pfeiffer, communications director of Maga Inc, said in a statement. “The people of Iowa sent a clear message tonight: Donald Trump will be the next Republican nominee for President.”
A DeSantis campaign spokesman accused the media of “election interference” for projecting a Trump victory before the voting had finished.
“The media is in the tank for Trump and this is the most egregious example yet,” Andrew Romeo wrote on X.
But other Republicans including Mike Johnson, Speaker of the US House of Representatives, congratulated Mr Trump.
“Tonight’s vote was an endorsement of President Trump’s extraordinary record and his ongoing vision to fix our economy, secure our border, and defend the freedoms and values that make America great,” the Louisiana congressman said.
Mr Trump, the runaway frontrunner nationally in the Republican race, had been the overwhelming favourite to win Iowa.
His dominance in the Midwestern rural state has drawn much of the focus from political analysts to the race for second place.
Iowa is widely seen as make-or-break for Florida Governor DeSantis, who barnstormed all 99 counties, throwing most of his time and resources into this contest as he courted evangelical voters.
A third-place finish here could spell doom for his campaign.
Ms Haley, aiming to be the first female US president, has enjoyed a boost in support in the wake of headline-making debate performances.
Some anti-Trump Republicans have been coalescing around the former US ambassador to the UN as the only viable alternative to the former president.
Hallie Still-Caris, who attended a caucus at Roosevelt High School in Des Moines, said Ms Haley had a better chance to win in November.
“I look at the other Republican candidates and President Biden, and I think she’s electable. That’s what we need – less drama and less chaos.”
As a sense of expectation has grown around Ms Haley, both Mr Trump and Mr DeSantis have turned their fire on the former South Carolina governor.
Mr Trump has consolidated his lead in national opinion polls even as he faces four separate criminal trials.
He also faces a judgement as soon as this month in a civil fraud trial that threatens his New York property empire. On Tuesday he is expected to attend another trial that will work out how much he should pay in defamation damages to E Jean Carroll, a woman he was found liable of sexually abusing.
After Iowa, the Republican primary race barrels next week to the state of New Hampshire, before voters in Nevada and South Carolina have their say in early February.
More than a dozen states vote in a single day in early March on so-called Super Tuesday.
The eventual Republican White House candidate will be confirmed at the party convention in July.
Iowa’s early spot on the campaign calendar presents contenders with a launchpad to build momentum. However, the Hawkeye State is not always a reliable bellwether.
The Republican winner of Iowa’s caucuses has not gone on to win the White House nomination in the last three competitive races: 2008, 2012 and 2016.
The results of the primaries and caucuses help determine how many delegates from each state will be sent to the national party conventions this summer.
To win the nomination, a Republican needs 1,215 delegates – out of more than 2,400 in total.
In Iowa, just 40 are up for grabs.
