Iowa caucuses: Republicans set for 2012 caucuses
Six contenders for the US Republican presidential nomination are making frantic final appeals to voters ahead of caucuses across the state of Iowa.
Tuesday evening's contest launches six months of state-by-state caucus and primary ballots.
Mitt Romney, Ron Paul and Rick Santorum lead in a race that remains too close to call. Many voters remain undecided.
The caucuses will involve about 120,000 Iowans gathering in homes, schools and public buildings.
Voters will gather at 20:00 EST (01:00 GMT) for sessions that last two hours.
The caucuses will take place in more than 1,700 locations in all of the mid-western state's 99 counties.
Iowans will elect 28 delegates to the Republican National Convention, where the party's eventual nominee will be anointed in Florida this August.
Iowa is not expected to settle the contest - John McCain, the eventual Republican nominee in 2008, came fourth in the state's caucuses that year - but it will help shape the race for the White House.
Although the candidates have been campaigning for months on end, about 40% of voters say they could still be swayed, and the Republicans made last-ditch efforts on Tuesday to win over the many undecided voters.
Ron Paul, Rick Santorum, Michele Bachmann and two of Mitt Romney's sons spoke at an afternoon "Rock the Caucus" event at a West Des Moines high school.
Mr Paul told a crowd of young voters he did not believe in sending young people to war unless "absolutely necessary".
Mr Santorum told the students that government would "crush your pocketbooks", the Des Moines Register reported.
Mrs Bachmann is now travelling to Black Hawk County, where she was born, to speak at a caucus event at 17:30 CST (23:30 GMT).
'Hard to predict'
Former Massachusetts Governor Romney, whose last run for the presidency in 2008 was effectively derailed in Iowa, adopted a more cautious tone on Tuesday, a day after predicting he would win.
He told MSNBC television: "It's hard to predict exactly what's going to happen. I think I'll be among the top group."
Recent opinion polls suggest a knife-edge contest between Mr Romney and Texas Congressman Ron Paul, a small-government, anti-war libertarian, who enjoys widespread support in the Hawkeye state.
But former Pennsylvania Senator Rick Santorum - whose conservative Christian message plays well in Iowa - has been closing the gap on both men.
Former House Speaker Newt Gingrich, who was until last month the front-runner in Iowa, hit back at Mr Romney on Tuesday.
He said Mr Romney should "just level with the American people" about his moderate political views.
Mr Gingrich also said it was "baloney" that Mr Romney had nothing to do with a political action committee that has pummelled Mr Gingrich with attack ads.
Asked on CBS television if he was calling Mr Romney "a liar," the former House speaker said: "Yes."
Mr Romney lost Iowa in 2008 amid voter scepticism over his Mormon faith and conservative credentials.
But he can win this time, analysts say, if the evangelical Christian vote is split across competing conservative candidates - including Mr Santorum, Texas Governor Rick Perry and Minnesota Congresswoman Michele Bachmann.
Iowa experts say the state's weather could be a determining factor in voter turnout on Tuesday, with Mr Paul's dedicated supporters the most likely to brave poor conditions.
The forecast for Iowa on Tuesday is partly cloudy, BBC Weather says, with temperatures likely to hover just above freezing during the day.
After Iowa, the state of New Hampshire holds its primary election on 10 January. Mitt Romney has a big lead there.
Over the next six months, each US state will vote on the presidential contenders before a final nominee is selected.
The candidate will run in the 6 November general election against Democratic President Obama, who is seeking a second term.
Mr Obama, meanwhile, plans to host a web-chat with supporters in Iowa on Tuesday as the caucuses take place.
Voters remain concerned by the slow pace of economic recovery from the recession that started during the end of the presidency of George W Bush and officially ended in 2009.