Republican Ted Cruz is holding a slight advantage over Donald Trump while Democrat Hillary Clinton is holding a small lead over rival Bernie Sanders in Iowa's presidential nominating contest, the first in the 2016 race for the White House.
Cruz, a conservative senator from Texas, led the billionaire businessman 27.9 percent to 24.5 percent in the Republican race with 76 percent of the precincts reporting results.
Marco Rubio, a senator from Florida, was in third place with 22.7 percent, making him easily the leader among establishment candidates.
Clinton, a former secretary of state, held a lead of 50.4 percent in the Democratic race compared to 49.0 percent for Sanders, a self-styled democratic socialist senator from the state of Vermont, with 64 percent of the precincts reporting.
| Voting to begin in key US state of Iowa
Earlier, US presidential candidates descended on Iowa to campaign for their party nominations in the first voting contest of the 2016 elections.
Al Jazeera's Kimberly Halkett, reporting from Des Moines in Iowa, said candidates spent the final hours holding rallies and urging supporters to come out and caucus.
"Turnout is critical for all the candidates. That's why over the weekend, the Clinton campaign reached out to more than 125,000 households," she said.
"Clinton is struggling to overcome issues of trust and a scandal involving her use of a private email server while Secretary of State."
Halkett said while a win in Iowa of the Democratic or Republican caucuses doesn't always point to an eventual win of the nomination, it provides important momentum and and drives donations for further campaigning.
"Historically a win is important for another reason too. No candidate who has finished less than third place has ever gone on to win the White House," she said.
The final major poll of the Iowa campaign, by The Des Moines Register and Bloomberg, found Trump to be leading the Republican race, although many Republicans staunchly oppose the front-runner.
Other top candidates have less support - but they are also less unpopular. Meaning, Trump is the most polarising contender.
Several findings point to this conclusion. First, just nearly half of likely Republican caucus-goers have a favourable opinion of Trump, while 70 percent like Rubio and 65 percent like Cruz.
READ MORE: US elections and 'horse race journalism'
Trump's unfavourable rating (47 percent) is surpassed only by that for Jeb Bush (53 percent).
Also, more would be enthusiastic about Rubio winning the nomination (58 percent,) or Cruz (56 percent), than Trump (44 percent).
Still, a vote can only go to one candidate and Trump is the top choice overall, if the poll is right.
It finds 28 percent for Trump, 23 percent for Cruz, 15 percent for Rubio and the others trailing. Clinton and Sanders are separated by only 3 percentage points, within the margin of error, on the Democratic side.
Among those striving for the Republican party's nomination are two previous Iowa winners who had little apparent success in this Iowa campaign - Mike Huckabee (2008) and Rick Santorum (2012) - and soon may be facing the end of the road.
Two of their partners in the undercard debates, Chris Christie and Rand Paul, clawed their way back to the main debate stage to make their pitch to Iowans and the nation in prime time.
Jeb Bush and Ben Carson, both trying to revive what once looked like promising campaigns, are also in contention, as is Carly Fiorina, who briefly attracted interest early on.
On the other side, former Maryland Governor Martin O'Malley was considered the most endangered Democrat.
Sources close to his campaign told Al Jazeera that O'Malley was expected to suspend his campaign after the Iowa caucuses.
A few departures may happen after Iowa, but New Hampshire on February 9 may be more of a winnowing event, because that is where more of the hopefuls are pinning hopes.
Source: Al Jazeera and agencies