Republican presidential candidates are out making their final pitch to voters on the last full day of campaigning before the Iowa caucuses.
Three contenders, Mitt Romney, Ron Paul and Rick Santorum, lead the pack ahead of Tuesday's vote.
But recent polls indicate many Iowans have still not decided who they want to vote for.
Iowa's ballot is the first contest to pick the Republican who will challenge Barack Obama for the White House.
Tuesday evening's caucuses will involve about 120,000 Iowans gathering in homes, schools and public buildings.
Candidates are meanwhile conducting a last-minute flurry of campaign events at coffee shops, pizza restaurants and hotel lobbies to win over undecided voters.
Mr Romney leads in the state, which derailed his last run for the presidency in 2008 amid voter concerns about his Mormon faith and perceived inconsistency on some social issues.
The former Massachusetts governor will win, analysts say, if the evangelical Christian vote is fragmented across competing conservative candidates.
An opinion poll on Sunday by the Des Moines Register newspaper suggested Mr Romney remained the most popular candidate in Iowa, with 25% support. Texas Congressman Paul was at 22%, while Mr Santorum scored 15%.
Former Pennsylvania Senator Santorum, who pitches himself as a social conservative and has surged into third place in the last week, on Monday took a swipe at multimillionaire Mr Romney.
Noting Mr Romney's emphasis on his business experience, he said: "We are not looking for executive experience. We are looking for a commander in chief."
Mr Santorum warned that choosing a candidate based purely on electability might prove to be a "pyrrhic victory".
He has campaigned hard in every one of Iowa's 99 counties, impressing the state's social conservatives with his message of rejecting gay marriage and abortion, even in cases of rape.
On Sunday, Mr Romney criticised Mr Santorum for the first time on the campaign trail, saying the former senator had "spent his career in the government in Washington".
After spending the holiday weekend in Texas, Mr Paul - at 76, the oldest candidate in the race - is back in Iowa.
The libertarian-leaning candidate has faced scrutiny over racially charged newsletters published in his name during the 1980s and 1990s.
He proposes immediately cutting $1 trillion from the budget and abolishing a number of government agencies, including the Department of Education. He also opposes intervention in Iran over its alleged nuclear weapons programme and wants to end all US foreign aid, including to Israel.
Former House Speaker Gingrich, who was knocked off his front-runner perch by a fusillade of attack ads, is trying to claw his way back into the race.
He is trailing at the back of the field with Texas Governor Rick Perry and Minnesota Congresswoman Michele Bachmann.
At least $12.5m (£8.1m) has been spent on largely negative political advertising in Iowa in recent weeks.
After Iowa, the state of New Hampshire holds its primary election on 10 January. Mitt Romney has a big lead there.
Former Utah Governor Jon Hunstman, who is also a Mormon, has skipped Iowa to campaign intensely in the Granite State.
Over the next six months each US state will vote on the presidential contenders before a final nominee is selected.
The eventual Republican nominee will be anointed at the party convention in August before running in the 6 November general election against Democratic President Obama, who is seeking a second term.
Voters remain concerned by the pace of economic recovery from the recession that started during the end of the presidency of George W Bush and ended in 2009.