Republican White House hopefuls have made their last-ditch pitches to woo voters across the central US state of Iowa, with polls giving at least three candidates a shot at winning the first contest of the 2012 presidential campaign.
Polls show the race in Iowa has turned into a contest among front-runner Mitt Romney, Ron Paul and Rick Santorum, who is quickly gaining ground with voters in the state.
Yet, Iowa's quirky caucuses are known more for weeding out candidates than picking the future president, and finishing in one of the top spots could provide a big boost to any contender in the volatile contest to pick a challenger to President Barack Obama in the November 6 election.
"You can't win the presidential nomination here in Iowa, but you can lose it," said Al Jazeera's Alan Fisher, reporting from Des Moines, Iowa.
"What happens here will shape the agenda of the Republican Party as they go next to New Hampshire and then South Carolina."
The tight Republican race, marked by ups and downs in opinion polls for most of the candidates, has sparked weeks of negative campaigning in the Iowa competition.
"It's hard to predict exactly what's going to happen. I think I'll be among the top group,'' Romney told MSNBC television on Tuesday, apparently backing off from his confident prediction at a Monday rally: "We're going to win this thing."
He is virtually tied in the polls with Paul, a small-government libertarian who has found fertile ground in Iowa for his states' rights, anti-war message.
Former Pennsylvania senator Santorum is gaining ground on both men, playing heavily on his conservative Christian, anti-abortion record with like-minded Republican voters in the heartland state, which is largely farmland and more than 91 per cent white.
Romney may withstand the challenges because Republicans see him as the candidate most likely to defeat Obama. The president is vulnerable because of the struggling American economy and continuing high unemployment as the country has been slow to rebound from the recession of 2007 to 2009.
In a fluid race that has elevated and then discarded a dizzying assortment of front-runners, many of Iowa's Republican voters still hadn't settled on a favourite candidate. More than one-third of all potential caucus-goers said they could yet change their minds.
Voters will gather at 0100 GMT for sessions that last two hours. In the living rooms of private homes, school auditoriums and libraries, they will hear from the candidates' local representatives for a final sales pitch before they write down the name of their favourite on blank pieces of paper handed out to each caucus-goer.
While just a small percentage of Iowans will attend the caucuses, around 120,000 Republicans showed up in 2008, and that was a record attendance, the state vote holds out-sized importance in the nominating process.
It is the first test of the candidates' popularity and ability to organise. The latter is particularly important as the nominating campaign stretches across primary elections and caucuses in all 50 states and is not finished until late June.
Twenty-five delegates are at stake in Iowa, out of 1,144 needed to win the Republican nomination in August. The delegates are not assigned until the state Republican convention on June 16.
Texas Governor Rick Perry, former House Speaker Newt Gingrich and Minnesota Representative Michele Bachmann all trailed the top three Iowa candidates.
But Gingrich was on the attack on Tuesday morning, calling on Romney to "just level with the American people" about his moderate political views. Asked on CBS television if he was calling Romney "a liar", the former House speaker replied, "Yes".
Neither Santorum nor Paul is likely to be as serious a challenge to Romney nationally as would Perry and Gingrich, who have both fallen in polls in recent weeks.
Romney faces the same challenge he did in 2008: winning over a conservative base that's uncomfortable with his moderate past. In 2008,socially conservative voters denied Romney a first-place finish, contributing to his eventual defeat.
This time, Romney's trying to win Iowa by arguing he's the most electable candidate against Obama, a pitch that is winning over conservatives who desperately want to beat the president.