Democratic and Republican presidential hopefuls are undergoing their first real test of public opinion as the US election process begins in Iowa.
Candidates have made final appeals for support, with both the Republican and Democratic races too close to call.
At caucuses, or local meetings, across Iowa, voters will decide which of the contenders to back for nomination.
The caucuses are now under way in schools and homes across the state with results expected within an hour or two.
Most opinion polls suggest an extremely tight three-way race between Democratic contenders Hillary Clinton, Barack Obama and John Edwards.
The latest survey prior to the caucuses by Reuters/C-SPAN/Zogby showed Senator Obama just ahead of Mr Edwards, with Senator Clinton in third place.
The same poll also showed Mike Huckabee extending his lead over his nearest rival Mitt Romney for the Republicans.
3 Jan: Iowa caucuses
8 Jan: New Hampshire primary
15 Jan: Michigan primary
19 Jan: Nevada caucuses; South Carolina primary (Rep)
26 Jan: South Carolina primary (Dem)
29 Jan: Florida primary
5 Feb: some 20 states including California, New York, New Jersey
After months of being courted by the politicians and surveyed by the polling organisations, Iowa voters now have their say as the contest begins in earnest.
But despite the nationwide impact of the caucuses, only about 200,000 people - from a state population of 3m - are expected to turn out on Thursday evening.
They will gather at more than 1,780 locations across the state to choose delegates, the start of a selection process that culminates in the Democratic and Republican nominating conventions in summer.
Iowa's Republican caucuses, which use straw polls, are relatively straightforward so the result is expected from around 0130GMT.
At the Democratic caucuses, voters publicly divide into groups, gathering in different corners of a room to show their support for the different candidates, and delegates are allocated accordingly.
The result of this contest is expected from 0300GMT on Friday.
Given the closeness of the vote, rivals chased the last wavering voters in a bid to take Iowa and so give their campaigns crucial momentum.
"Walk quick, talk fast," said Barack Obama to his supporters as they tried to drum up support for him in the winter cold.
Mike Huckabee has shot up the opinion polls in recent weeks
"I feel good but it depends on who comes out, who decides to actually put on their coats, warm up their cars and go the caucuses," said Mrs Clinton.
Mr Romney, who has spent millions campaigning in Iowa, damped down expectations, saying he would settle for second place, the Associated Press reported.
But not all contenders were focused on Iowa. Republican Rudy Giuliani, who trails well behind in the Iowa polls but who has been leading nationally, was in Florida on Thursday, making a bid for that state's nomination when it holds its primary on 29 January.
The Iowa caucuses and the New Hampshire primary on 8 January are regarded as key for building momentum in the state-by-state process of winning the presidential nomination.
Candidates who do poorly tend to drop out of the race.
Polls from New Hampshire have indicated that the Republican contest is between Mr Romney and Senator John McCain, while for the Democrats Mrs Clinton and Mr Obama lead the field.