Judgment day dawned for Republican contenders vying to take on Barack Obama in November's presidential election, with a pivotal vote in Iowa Tuesday likely to dramatically winnow the field of contenders.

The mostly rural heartland state's caucus is the first formal vote of the war for the White House against the backdrop of a sour, job-hungry US economy that weighs heavily on the embattled Obama's bid for a second term.

Frontrunner Mitt Romney on Tuesday focused his last minute attacks on the Democratic president, amid polls suggesting good chances of a win here.

"President Obama failed because he'd never been a leader anywhere. He never led in the state Senate. He never led in the US Senate. Never led an enterprise of any kind," Romney told NBC television's "Today Show" program.

"His lack of experience is not just a lack of experience in foreign policy, it's a lack of experience as a leader," Romney said early Tuesday.

For the 64-year old former Massachusetts governor and millionaire venture capitalist, the Iowa vote offers a chance to set in motion a series of early primary victories that could see him nab the nomination with relative ease.

But with nearly half of Iowa voters saying they are still up making up their minds, he has only a narrow lead over veteran Representative Ron Paul of Texas. Former senator and ardent social conservative Rick Santorum has surged to third place.

"This is what the vote is about tomorrow: Are we sick and tired of the expansion of government?" Paul told a rowdy crowd packed into a hotel ballroom in Des Moines on Monday, calling his rivals agents of the "status quo."

Paul, 76, a small-government champion opposed to foreign aid and military interventions overseas, accused the other candidates of supporting a foreign policy of "mischief around the world and policing around the world."

"This is my guy," emergency room doctor Healy Burnham, 64, told AFP, as he smoothed out a Christmas-themed necktie. "How many foreign wars do we need?"

Come November, "if it's Romney or Obama, I'll vote for Romney," he said.

Santorum pleaded with Iowans not to "settle" for Romney, and "to pick the more conservative of the candidates" as he, too, attacked Obama for "failing this country."

He also fought back tears at a stop in Newton, Iowa, where he was asked about a 1996 family tragedy when the Santorums had a premature baby boy who died just hours after birth, calling it "a very difficult time in our life."

"Rick Santorum is a real conservative, I feel like I know his values," said Richard Puhl, 45, a Des Moines computer programmer who brought his children, aged four and seven, to see Santorum at a Pizza Ranch restaurant in Boone.

What about Romney versus Obama? Puhl didn't hesitate: "Romney. But I'd prefer not to have that choice."

Paul's unorthodox libertarian views have earned him a devout following, but he is seen as uncompetitive in other states, while Santorum, 53, faces an uphill fight to match Romney's massive national organization.

Public opinion polls have been crueler to former House speaker Newt Gingrich, Texas Governor Rick Perry and Representative Michele Bachmann, who said Sunday she hoped for a "miracle" in the heartland state's caucuses.

All of the candidates planned a frenetic last campaign dash, mindful that the winning margin in 2008 was 10,000 votes out of just 120,000 cast -- a fraction of Iowa's two million registered voters.

Iowans, some of whom have met all seven candidates to stump here, will meet in school cafeterias and gymnasiums, church buildings and other spots to vote after hearing speeches from their neighbors on behalf of the candidates.

Unpredictable Iowa -- where unemployment is well below the national average -- is also an unreliable predictor of presidential fortunes: Senator John McCain, the eventual nominee in 2008, came in fourth that year.

But a victory in Iowa can lift a sagging campaign or give a top contender an air of inevitability, bringing fundraising dollars, endorsements and voter support that can shape the rest of the state-by-state nominating battle.

Romney's massive campaign war chest and high-profile endorsements have fed his image as the candidate to beat -- but he faces stubborn doubts about his conservative credentials and has been unable to increase his support among Republican voters nationwide above 30 percent.

The Weather Channel forecast relatively mild temperatures, with only a 10 percent chance of rain, setting the stage for a high turnout.

That could help Romney -- seen as having more lukewarm supporters -- and hurt Paul, by diluting the impact of his relatively small but dedicated following.