Republican White House hopefuls are making their final pitches to voters as they enter a last day of campaigning ahead of Tuesday's Iowa caucus.
Mitt Romney maintains a narrow lead over Ron Paul, his nearest rival, although opinion polls show many in the state remain undecided.
Meanwhile, former US senator and social conservative Rick Santorum surged ahead into third place.
The vote is the first major test of the 2012 presidential election.
It marks the start of the six-month period during which each US state will hold primary elections or caucuses to pick a Republican candidate, who will be officially nominated at the party convention in August.
An opinion poll on Sunday by the Des Moines Register newspaper suggests Mr Romney remains the most popular candidate in Iowa, with 25% support. Mr Paul was at 22%, while Mr Santorum scored 15% but rising.
Former House Speaker Newt Gingrich came in fourth with 12%, while Texas Governor Rick Perry got 11% and Minnesota Representative Michele Bachmann had 7% support.
However, 41% of those polled said they could still change their minds.
"It's a wide open race," Iowa Governor Terry Branstad, a Republican, told CNN, adding that many were still on the search for their ideal candidates.
"I'm pretty confident we'll have a good night. I don't know who's going to win," Mr Romney told supporters at a restaurant in the town of Atlantic, says the Reuters news agency.
Mr Romney, a former governor of Massachusetts and a moderate Republican, had barely campaigned in Iowa until the past week.
He spent millions on his unsuccessful nomination bid in 2008.
Support for Texas Congressman Ron Paul has slipped a little after he was challenged about his non-interventionist foreign policy views.
"I may come in first, I may come in second. I doubt I'll come in third or fourth," he told CNN.
Mr Paul opposes intervention in Iran over the issue of nuclear weapons and wants to end US aid to Israel and other allies.
Standing firm, he told CNN: "I would say that we just need to be more cautious. I think if we overreact and participate in bombing Iran, we're looking for a lot more trouble."
Mr Santorum, meanwhile, has said he would bomb nuclear facilities in Iran if they did not allow international arms inspections.
Correspondents say that while a good performance in Iowa does not guarantee an easy path to nomination, a successful result can give a clear boost to candidates as they start out. Equally, a clear defeat could spell the end for a contender.
The eventual Republican candidate will challenge President Barack Obama for the White House in November 2012.
Many voters are concerned by the pace of economic recovery from the recession that started during the last months of the presidency of George W Bush and ended in 2009.