The Republican White House hopefuls have launched a final spring to the finish in the central state of Iowa, with front-runner Mitt Romney poised for a strong showing that could set him on the path to the nomination.
Romney holds a slight edge over rival Ron Paul in recent polls in Iowa, which holds the first contest in the state-by-state battle to pick a challenger to President Barack Obama in November.
"I'm pretty confident we'll have a good night. I don't know who's going to win," Romney told supporters at a packed restaurant in Atlantic, Iowa, adding he was "energised" ahead of Tuesday's contest.
Even a strong second-place showing in Iowa would be good news for the former Massachusetts governor.
Paul could have trouble competing with him in later contests in New Hampshire, where Romney leads in polls, and in other states.
Romney, who spent millions in Iowa in 2008 only to lose to former Arkansas Governor Mike Huckabee, did not campaign hard in the state until the last week.
He picked up the endorsement on Sunday of Iowa's Quad-City Times newspaper, which praised his business background as a former head of a private equity firm and said he had the best chance of beating Obama.
Rick Santorum, a former US senator, has surged past Newt Gingrich into third place in polls, building momentum in the final days of a close race that has seen a series of candidates rise and fall.
He urged supporters at a coffee shop in Sioux City to send a "shock wave" across the country by giving him an upset win in Iowa.
"Lead this country. That's what I ask the people of Iowa. Lead, don't defer," Santorum said. "Don't put forward somebody who isn't good enough to do what is necessary to change this country."
Gingrich, the former House speaker who has dropped in Iowa polls after an onslaught of attack ads from Paul and a group that backs Romney, said he would stay in the race no matter where he finishes in Iowa.
Asked in a Reuters interview on Sunday whether coming in fourth or lower would make him consider dropping out, Gingrich said, "No".
Gingrich said he had enough campaign funds to get him through New Hampshire and on to conservative South Carolina, which comes next on January 21.
"By the time we get to South Carolina, it will be very clear the gap between a Massachusetts moderate who hides his record behind negative ads and a conservative who's talking about positive ideas," Gingrich said.
Rallying the base
Michele Bachmann, who could face the end of the line if she does badly in Iowa, went to church on Sunday morning to woo the critical Christian conservative vote, which has been split among her, Santorum, Gingrich and Texas Governor Rick Perry.
Bachmann has sunk to the bottom of polls and is beset by a lack of money and staff desertions.
"On this January 1, 2012, I admonish you, don't for one moment think that your adversity is one that cannot be scaled," Bachmann told churchgoers at a service in Oskaloosa, making biblical references to underdog Israelites defeating their enemies.
Romney, who attended a Mormon church service in Iowa on Sunday morning, likely raised more than $20m in the final three months of 2011, a Republican source said.
That amount would almost certainly put him far in front of his Republican rivals and underscores the long-term advantage he has in organisation and money.
"We're looking better this quarter than any other quarter so far," Romney said, although he did not give a final number. He raised $14m in the third quarter.
A win in Iowa for Romney, combined with a victory in his stronghold of New Hampshire on January 10, could put him on a path to clinch the nomination early.
It would make him the first fRepublican who is not an incumbent president to win the party's first two contests.
"Romney just has to prove that he's conservative enough for me," said Eleanor Stump, a 70-year-old Tea Party member from Sheldon, Iowa. "I don't like the way he's flip-flopped."
Romney is distrusted by some conservatives who remember his past support for abortion rights and for a state health care plan similar to Obama's federal overhaul.
Stump said she initially supported Herman Cain, who dropped out of the race after charges of an extramarital affair, then went to Perry, then back to Cain and then to Gingrich.
"I've gone back and forth so many times," she said.