Donald Trump and his Republican rivals are making last-ditch pitches to win support in Iowa, hours before voters in the state will kick-off the 2024 race for the White House.
Candidates are holding final events, but frigid conditions have complicated the last days of campaigning.
A resounding victory in Iowa would cement Mr Trump's frontrunner status.
His rivals, meanwhile, are seeking to establish themselves as the main alternative to the former president.
Republican voters will meet at one of more than 1,500 caucus locations around the Midwestern state on Monday night to state their preferred presidential candidate. All of the candidates have urged voters to brave the extreme cold, as temperatures are forecast to fall as low as -30C (-20F), so they can have their say in the contest amid fears the weather could harm turnout.
The race will then move state by state before an eventual nominee is selected to almost certainly challenge Joe Biden in November's general election.
Iowa has a patchy record of selecting the eventual Republican nominee, and has not done so since 2000 when voters in the state backed George W Bush.
Mr Trump held a rally in the city of Indianola on Sunday and called for his supporters to turn out. "Together we're going to make history but you have to show up," he said. "The outcome in this state will send a message to the entire country and, in fact, the entire world."
The former president, 77, is seeking to land a knockout blow on his challengers and win by a big margin. "We're looking to set records," he said during a virtual rally on Friday.
His campaign has relied heavily on its ground game, with regular "commit to caucus" events serving as trial runs for the real deal on Monday. These events, where Iowans are recruited by door knocking "caucus captains", often include an animated video on how to caucus, a sign of how Mr Trump is hoping to mobilise first-time voters and win big.
The Trump campaign's ambition of a big win was boosted on Saturday evening by a final poll from the Des Moines Register/NBC News/Mediacom that suggested he had a nearly 30-point lead.
The closely watched poll showed Nikki Haley, the former US ambassador to the UN, had moved into second place after gaining momentum in recent days.
Florida Governor Ron DeSantis, who has invested large amounts of resources into Iowa, had slipped to third place. Mr DeSantis will face pressure to drop out if he performs poorly on Monday and the result could prove critical for his campaign.
Ms Haley, 51, sought to downplay the results shortly after, saying the "real poll" was on caucus day. "We just want to come out of Iowa strong," she said.
A strong finish in the state would give the Haley campaign crucial momentum heading into the next contest in New Hampshire, where she is polling within 10 points of Mr Trump. After that, the next contest is in her home state of South Carolina where she served two terms as governor.
A win in one of these early states would help establish Ms Haley as the only viable alternative to the former president, likely triggering a much-needed wave of support and donations.
In the final sprint around Iowa, Ms Haley doubled down on her pitch for change, urging voters to leave the "chaos" of Mr Trump behind. "This comes down to a choice," she told supporters in Cedar Falls. "You've got the opportunity to look back at the past and continue, or go forward and start new."
Facing stiff competition for second place, Mr DeSantis has taken an aggressive approach to the final days of the campaign, attacking both Ms Haley and Mr Trump. "Donald Trump is running for his issues. Nikki Haley is running for her donor's issues," he often says at the beginning of his events. "I'm running for your issues."
Mr DeSantis has kept up an intense schedule of events, which he and campaign staff are quick to point out have taken place even as the other candidates have cancelled appearances because of the winter storm. He has toured all 99 counties and spent considerable time in Iowa in recent weeks, focusing on organising an effective ground campaign.
The Florida governor has repeatedly questioned the accuracy of the polls that show him in third place. "I think it's very hard to poll an Iowa caucus," he said during an interview with CNN on Sunday. "The [2016] poll was not accurate and predictive, but especially one in -20 degrees."
"I know the media has been saying that [Trump] is unstoppable," he said. "Ultimately, people are going to be able to make a decision... there's a lot of voters who haven't made a final decision."
Elsewhere, in the east of the state, Vivek Ramaswamy held an event on Sunday in front of dozens of people. The biotech entrepreneur, 38, is polling in fourth place and has increasingly spoken about conspiracy theories in recent weeks, notably airing false claims of election fraud in 2020.
He defended Mr Trump at the event, who attacked him publicly for the first time on Saturday, and blamed Mr Trump's comments on his advisers. He also said he was experiencing a "late surge" in support.
"This is sort of a 'prove it' moment for all these other candidates," Jimmy Centers, an Iowan Republican consultant, told the OceanNewsUK. "You say the Republican Party would be better suited to go a different way, now voters will have their say."
