The Des Moines Register/NBC News poll released late on Saturday gave Trump 48%, though his lead slipped by 3 percentage points from the December poll. Former South Carolina Governor Haley was second at 20%, up 4 points, followed by Florida Governor DeSantis with 16%, down 3 points.
Organizers of the poll pride themselves on taking an accurate snapshot of the mood of Iowa voters.
The survey provided a strong indication that Trump was poised to do well in Monday's caucuses, the first voting contest on the road to the presidential election in November.
A victory would give Trump, 77, an important big step toward securing the Republican nomination to face the Democratic incumbent, Joe Biden, 81, in a rematch of their 2020 battle. It would be evidence that many Republicans are willing to look past Trump's mounting legal problems and inflammatory rhetoric.
Trump plans a Sunday rally in Indianola, a suburb of the capital Des Moines, but the weather forced him to cancel one in the city of Cherokee. The former president canceled two events on Saturday, appearing in the evening on a campaign livestream.
"We are leading by a lot in all the polls and you have to get out," Trump told his supporters. "We need to send a message most importantly for November."
DeSantis and Haley, who was Trump's U.N. ambassador, both need to do well enough in Iowa to give them some momentum for the next contest, in New Hampshire on Jan. 23.
The DeSantis campaign predicted a strong performance on Monday night, despite the slip in the widely watched poll.
"Winning campaigns don't rely on public data. Most importantly, no one has worked harder and is better organized than Ron DeSantis," spokesman Andrew Romeo said.
Their battle increasingly bitter, Haley and DeSantis begin Sunday in Dubuque in the east of the state near the Mississippi River, followed by another DeSantis event around 300 miles (500 km) away in Sioux City.
From 7 p.m. CST on Monday (0100 GMT on Tuesday), Iowans will gather for two hours in school gymnasiums, bars and other locations to debate the Republican candidates before ranking them in order of preference.
Results are typically announced within a few hours.
With temperatures plunging, one question facing the candidates was whether their supporters would be sufficiently motivated to brave the cold and show up to caucus.
Forecasters predict the coldest Iowa caucus night ever - minus 20 degrees Fahrenheit (minus 30 degrees Celsius).
DeSantis, speaking at an event in West Des Moines on Saturday, urged people to make the effort.
"It's not going to be pleasant," he said. "But if you're willing to go out there and you're willing to fight for me... then as president I'll be fighting for you for the next eight years."
Voters like Michelle Mahoney, a 58-year-old businesswoman from West Des Moines, gave hope to the Trump challengers.
She said she voted for Trump twice but finds the former president divisive and plans to caucus for either Haley or DeSantis.
"I'd be thrilled if we could have DeSantis or Haley," she said at a DeSantis campaign event.
Mahoney likes DeSantis' military service and says his leadership during Hurricane Ian in 2022 was "fabulous." She views Haley as a unifier.
Trump, president from 2017 to 2021, was doing well despite a host of issues.
He continues to claim falsely that his 2020 loss to Biden was due to widespread fraud and has vowed if elected again to punish his political enemies, introduce new tariffs and end the Ukraine-Russia war in 24 hours, without saying how, according to his own comments, those of his campaign and media reports.
He has drawn criticism for increasingly authoritarian language that has echoes of Nazi rhetoric, including comments that undocumented immigrants were "poisoning the blood of our country."
Trump has used charges of unlawfully trying to overturn his 2020 election loss to fundraise and boost his support among Republican voters and elsewhere, and claim a "witch hunt" as he protests his innocence.
He faces four prosecutions, setting up the unprecedented prospect of a president being convicted or even serving from behind bars, with the courts almost certainly weighing in at every stage.
DeSantis, who has tacked to the right of Trump especially on issues such as education and LGBTQ rights, has staked a huge amount on a strong performance in Iowa with associates of his saying he needs to finish at least second.
(Reporting by Nathan Layne, Gabriella Border and Tim Reid; Writing by Steve Holland; Editing by William Mallard)
By Nathan Layne, Gabriella Borter and Tim Reid
