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 Republican Donald Trump has a narrow lead over Ted Cruz but both are well ahead of the others.
But the Democratic race is slightly tighter, with former US Secretary of State Hillary Clinton 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.
Among 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.
Iowa has an unusual election system called a caucus, which involves people gathering at sites across the state at 19:00 local time (01:00 GMT Tuesday).
Democratic voters divide themselves into groups based on their preferred candidate but the Republican caucus process is more like a traditional ballot.
The last Republican winner in Iowa who won the party’s nomination was George W Bush 16 years ago.
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.
Candidates are worried that the incoming storm could prevent their voters turning out earlier in the evening.
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.