US presidential hopefuls are to make final pitches to people in Iowa, where the first votes for party nominations will take place later 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 an attempt to court undecided voters.
Campaigning is expected to continue until voting starts at 19:00 local time on Monday (01:00 GMT on Tuesday).
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.