Former US Secretary of State Hillary Clinton is just edging ahead of Senator Bernie Sanders for the Democrats
The front runners in the opening round of the race to see which candidates run for US president are just a few percentage points apart from each other.
Opinion polls show that leading Democrat Hillary Clinton is supported by 47% of people likely to vote in Monday's Iowa caucuses, compared to her main rival Bernie Sanders' 44%.
Donald Trump, the Republican setting the early pace, is backed by 30% of his party's supporters, with Texas Senator Ted Cruz on 24% and Florida Senator Marco Rubio on 15%, RealClearPolitics found.
All the candidates spent the last day before voting on a last-ditch round of speeches, meetings and rallies.
Although the Iowa caucuses provide only a handful of the delegates that will vote at each party's convention in July, the first event in the series of caucuses and primaries is seen as an important chance to gauge the various candidates' support.
Those garnering the least number of supporters will be under pressure to drop out and leave the stronger candidates with a clearer run.
Mr Trump, who spent the final hours before party members gather campaigning with his wife Melania and heavily pregnant daughter Ivanka, looked like he could have caused a last-minute upset when he spelled the name of Iowa's second largest city wrong.
In giving his itinerary for Monday on Twitter, Mr Trump said he would be in "Ceder Rapids", rather than Cedar Rapids, at 1.30pm.
It prompted voter Billy Batts to tweet: "You spelled Cedar Rapids wrong, you dumbass."
Both Ted Cruz and Marco Rubio used some of their last speeches to attack Mr Trump and each other.
Hillary Clinton came on stage in Iowa's largest city, Des Moines, after an introduction by her husband the ex-president Bill Clinton.
She launched a veiled swipe at her possible Republican opponents by telling an audience the Democrats had run a "clean" campaign.
It comes after Mr Trump called for a ban on Muslims entering the US and said Mexicans were responsible for importing drugs, crime and rapists.
Her rival Bernie Sanders, a political outsider, urged his supporters to help him make history and send a message to those who back establishment politics.
Kicking off his final day of campaigning in Waterloo, he admitted he needed a large turnout if he had any chance of beating Mrs Clinton.
His call for his supporters to vote in large numbers came as it was suggested the weather could have an influence on the result.
Forecasters said a blizzard is due to hit the northern part of the state on Monday evening, arriving from the west in the evening.
The caucuses start at 7pm in 1,100 schools, churches and other public locations across the Midwestern state.
thern part of the state on Monday evening, arriving from the west in the evening. The caucuses start at 7pm.