One day before Iowa holds the first vote of the 2012 White House race, Republican candidates blitzed the state, with frontrunner Mitt Romney ripping President Barack Obama on the sour US economy.

"This American president didn't cause the recession, but he made it worse and it'll last longer because of his policies," the former Massachusetts governor and millionaire venture capitalist told cheering supporters in Clive.

Romney and his rivals criss-crossed Iowa on the eve of its first-in-the-nation Republican nominating vote, which opens a state-by-state battle to pick the party's standard-bearer in the November 6 elections.

Romney, 64, has taken a thin lead in polls over veteranRepresentative Ron Paul of Texas, while 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 Iowa's capital Des Moines, calling his rivals agents of the "status quo."

Paul, 76, whose libertarian leanings include staunch anti-interventionism overseas, accused the other candidates of supporting a foreign policy of "mischief around the world and policing around the world."

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

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

Santorum pleaded with Iowans not to "settle" for Romney, who is seen as the more potent general election candidate, 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 computer programmer from Des Moines, who brought his children, ages 4 and 7, 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."

Romney has made little secret that he hopes a first- or second-place showing in Iowa -- followed by a win in New Hampshire one week later, and a good score in South Carolina after that -- will let him win the nomination relatively early so he can focus his energies on the embattled Democratic president.

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.

But those long-shot candidates still raced across Iowa, its famed cornfields barren and brown with the winter chill, mindful that surveys have found nearly half of the state's voters say they could yet change their mind.

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 extra 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.

About 120,000 Iowans will gather Tuesday in hundreds of precincts across the state, meeting in school cafeterias, church buildings and other spots to vote after hearing speeches from their neighbors on behalf of the candidates.