Republican presidential contenders Mitt Romney, Ron Paul and Rick Santorum were locked in a tight battle as voters met across this largely rural state on Tuesday in the first contest to determine a challenger to Democratic President Barack Obama.
Partial returns from the state Republican Party showed the three candidates winning roughly 23 percent of the vote each with about 22 percent of precincts reporting.
The months-long campaign has been marked by volatility. Romney is a favourite of the party's business wing, while Santorum appeared to be consolidating the state's large bloc of Christian conservatives. Paul has drawn a passionate following among libertarians and younger voters.
Iowa's caucuses are known more for weeding out candidates than picking the future president but a strong finish here could provide a big boost in the state-by-state battle to choose the Republican to stand against Obama in the November 6 election.
Most of the candidates have topped opinion polls at one point in a race that until recently centred on televised debates rather than on-the-ground campaigning.
Many voters remain undecided and the unusual caucus process adds an element of unpredictability. Voters gathered in public meetings at hundreds of sites around the state such as schools, libraries and churches, listening to speeches touting the various candidates before casting their ballots. Democrats and independents are allowed to participate as long as they re-register as Republicans at the site.
I'm paying great attention, I just can't decide, said Judy Peters, the owner of an events center where roughly 1,000 voters were to meet. There's bits and pieces of each candidate that I like and bits and pieces that I don't.
Outside groups associated with candidates, known as Super PACs, have taken advantage of loosened campaign-finance rules to flood the Iowa airwaves with negative advertising.
Former House of Representatives Speaker Newt Gingrich has seen his support erode under a barrage of such attack advertisements. Early returns showed him in fourth place ahead of Texas Governor Rick Perry.
Poor Newt. I kind of feel sorry for him. He's just been savaged, said voter James Patterson, who said he plans to vote for Romney. Somebody's really, really mad at Newt.
Gingrich said he would keep his campaign positive.
You have a chance tonight to send a signal to America, he told voters in Cedar Falls. You can do that by refusing to vote for anyone who has run negative ads.
Sparsely populated Iowa only yields 28 delegates of the 1,143 needed to lock up the Republican presidential nomination, and those delegates aren't actually awarded for months after Tuesday's caucuses.
Still, the stakes are high.
Romney, a former Massachusetts governor, is aiming for a win that could ease persistent doubts among conservatives about his moderate past and propel him toward clinching the nomination early. He is heavily favoured to win next week's New Hampshire primary.
Surveys show Romney performs best among Republicans in head-to-head matchups with Obama in a campaign certain to focus on the economy and high unemployment.
Santorum, a former U.S. senator from Pennsylvania, hopes to emerge as the latest conservative alternative to Romney.
I was really not interested in any of the candidates at all and I didn't notice Santorum because he was so low in the polls, said Rachel Wright, an artist who spoke on behalf of Santorum at a caucus in Ames. She approved his anti-abortion stand. He is really strong pro-lifer. I admire his faith, and I almost admire that he isn't in our face about it.
Largely consigned to the margins for most of the race, Santorum is now fending off attacks from his rivals who see him as a new threat. On Tuesday, he accused Paul of launching a wave of automated phone calls that questioned Santorum's anti-abortion and pro-gun credentials.
A win by Paul, a congressman from Texas, would help him extend his minimal-government stance and broaden the appeal of his campaign outside his zealous base, many of them independents, disaffected Democrats and younger voters.
Struggling rivals like Texas Governor Rick Perry and U.S. Representative Michele Bachmann are fighting for at least a fourth-place finish that could preserve their flickering hopes.
People are looking for the one true core conservative that can take on Barack Obama and win. That's what I've demonstrated throughout the campaign, Bachmann told CNN.
The caucuses start a frenzied month for the Republican presidential hopefuls that will include a half-dozen debates in January and three more state votes -- on January 10 in New Hampshire, January 21 in South Carolina and January 31 in Florida.
Iowa's nominating contest has traditionally cleared the field of losers and elevated surprise contenders. Former Arkansas Governor Mike Huckabee won the 2008 Republican caucuses, but fell short of the nomination. The eventual nominee, Arizona Senator John McCain, finished a distant fourth.
Obama launched his White House run with an Iowa win four years ago. This time, Obama is the only Democrat running, but the party is holding caucuses anyway and he will address caucus-goers by video on Tuesday night.
(Additional reporting by John Whitesides, Steve Holland and Sam Youngman in Iowa and Lily Kuo in Washington; Writing by Andy Sullivan; Editing by David Storey)