US Republican candidates crisscrossed Iowa in a last-minute bid for support ahead of the first contest of the 2012 presidential campaign, and at least three - Mitt Romney, Rick Santorum and Ron Paul - appeared to have a shot at victory.
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Iowa's caucuses are known more for weeding out candidates than picking the future president.
Finishing in the top spot could provide a big boost in the state-by-state battle to select a Republican challenger to Democratic President Barack Obama.
Most of the candidates have topped opinion polls at one point in the volatile Republican race that until recently centred on televised debates rather than on-the-ground campaigning.
Polls show Mr Romney, the favourite of the party's business wing, in a tight race with Mr Paul, a US congressman from Texas with libertarian views, and Mr Santorum, a former US senator from Pennsylvania who hopes to consolidate Iowa's large bloc of Christian conservatives. Mr Romney is a former Massachusetts governor.
Many voters remain undecided. The unusual caucus process adds an element of unpredictability.
Voters in Iowa gather 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.
Outside groups loosely allied 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, who has seen his support erode under a barrage of such attack advertisements, lashed out at Romney for trying to distance himself from the ads.
Asked if he was calling Romney a liar on the CBS program "The Early Show", Mr Gingrich said: "Yes."
"This is a man whose staff created the PAC, his millionaire friends fund the PAC, he pretends he has nothing to do with the PAC. It's baloney," said Mr Gingrich.
Mr Romney said his campaign did not coordinate with the super PAC and told Mr Gingrich to toughen up.
"If you can't stand the heat of this little kitchen, wait for the hell's kitchen that's coming from Barack Obama," Mr Romney said on Fox News.
Sparsely populated Iowa only yields 28 delegates of the 1,143 needed to lock up the Republican presidential nomination, and those delegates are not actually awarded for months after Tuesday's caucuses.
Still, the stakes are high. A strong performance in Iowa would provide visibility and a surge in donations to the winner or winners at the start of what is expected to be the most expensive election in history.
