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.
Analysis - Jon Sopel, BBC North America editor, Iowa
On the Democratic 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
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.
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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.