Republican front-runner Donald Trump is set to put his precedent-shattering campaign to the test with voters in Iowa beginning the nationwide process of choosing a new US president.
Polls show a tight battle with Ted Cruz that could hinge on turnout and a large bloc of undecided voters.
Democratic front-runner Hillary Clinton also faces a stiff challenge in Iowa from insurgent Bernie Sanders in the first contest in the state-by-state battle to pick candidates for the November 8 election to succeed President Barack Obama.
Late polls showed Trump, the blunt-spoken billionaire businessman who has never before sought public office, with a small lead on Cruz, a conservative US senator from Texas, while Clinton had a slight edge on Sanders, a US senator from Vermont.
But uncertainty remained about who would turn up at the caucuses, and how successful Trump and Sanders would be at getting their supporters, many new to the process and disenchanted with politics as usual, to participate.
Adding to the unpredictability in Iowa was a large bloc of undecided or persuadable voters in both parties in a state where voters accustomed to a long courtship from candidates are traditionally in no rush to make a commitment.
The Des Moines Register/Bloomberg Iowa poll released on Saturday night showed three in 10 likely Democratic caucus-goers and 45 per cent of likely Republican caucus-goers were still uncertain and could be persuaded to switch to another candidate.
The vast array of choices along with the ability to see the candidates up close over the course of the past year gives many Iowa voters little incentive to rush into a firm choice.
"I'm still checking them out. The field is large and it requires some thought," said Paul Albritton of Carlisle, Iowa, a training co-ordinator at Iowa State University, as he waited to see US Senator Marco Rubio of Florida last week.
"I'm thinking about who can win in November."
For the winners in Iowa, the prize will be valuable momentum in nominating battles that could stretch for months, while many of the losers on the Republican side quickly could begin dropping by the wayside.
Iowans will attend caucuses at schools, libraries and other public locations on Monday evening, with results expected within hours.
They often make their decisions late. In 2012, nearly half of Republican caucus-goers, 46 per cent, decided in "the last few days", according to entrance polls of the participants.
In 2008, almost one in five Republicans decided on the day of the caucus and 13 per cent decided in the final three days, the entrance polls showed.
Democrats were only slightly more decisive in 2008, when 11 per cent decided the day of the caucus and 9 per cent decided in the last three days, according to entrance polls.
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