US presidential candidates are making a final pitch to people in the state of Iowa, where the first votes for party nominations will take place on Monday.
Polls suggest that business tycoon Donald Trump has a lead over Ted Cruz and the other Republican candidates.
But the Democratic race is much tighter, with former US Secretary of State Hillary Clinton just edging ahead of Vermont Senator Bernie Sanders.
Each party’s nominee will contest the presidential election in November.
Over the weekend, the candidates barnstormed the sparsely populated Midwestern state of Iowa in a last-minute attempt to court undecided voters.
Candidates are hoping to triumph in this first electoral test because victory can spark campaign momentum as voting moves to the other states.
Analysis – Jon Sopel, BBC North America editor, Iowa
On the Democratic Party side, Hillary Clinton is facing a fierce challenge from the new kid on the block, Bernie Sanders, the 74-year-old senator from Vermont.
He describes himself as a democratic socialist, he wants to raise taxes, he doesn’t argue with the description of himself as a grumpy old man – but he has been attracting thousands and thousands of people to his rallies, and millions and millions of dollars in donations.
His popularity seems to fly in the face of all conventional political wisdom. But then again, everything about this race so far has flown in the face of all the hoary old truths.
All eyes on Iowa in unpredictable race
Amongst the wide Republican field, recent polling suggests that businessman Mr Trump has a comfortable, though not certain, lead over his main rival, Texas Senator Mr Cruz.
The Democrats’ far smaller field – three candidates as opposed to 11 – appears to be more competitive.
Frontrunner Hillary Clinton has a commanding lead nationally but in Iowa she is narrowly ahead of self-proclaimed Democratic socialist Bernie Sanders.
- How does a US election work?
- What would a Trump presidency be like?
- How is Iowa like the Oscars?
- Brits and Americans quizzed on the jargon
One issue that could have implications in Iowa is the weather.
The National Weather Service is currently forecasting a winter storm to strike the area on Monday night.
23% Ted Cruz
15% Marco Rubio
10% Ben Carson
5% Rand Paul
42% Bernie Sanders
3% Martin O’Malley
Candidates are worried that the incoming storm could prevent their voters turning out earlier in the evening, when the caucuses are held at 19:00 local time.
Mr Trump joked with his supporters on Saturday, saying: “You’re from Iowa! Are you afraid of snow?”
Iowan law mandates that it be the first “state, territory, or any other group” to select delegates in the presidential nomination
This first vote in Iowa will be followed in the weeks ahead by more ballots in the 49 other states plus US territories.
Each party’s nominee will be chosen by the summer, and the US will pick its next president in November.