Republican presidential candidate Mitt Romney clung to a narrow lead in Iowa Sunday, two days before the heartland state casts the first votes of the 2012 election season.

Romney's massive campaign warchest and high-profile endorsements have fed his image as the candidate to beat in the state-by-state fight for the party's nomination to take on President Barack Obama in the November 2012 elections.

A Des Moines Register poll, made public by the newspaper Saturday, found that 24 percent of Iowans likely to take part in Tuesday's caucuses back the former Massachusetts governor and financier, the on-again off-again frontrunner over the past year.

But the study found veteran Texas Representative Ron Paul within striking distance at 22 percent, and former senator Rick Santorum, a fiery social conservative, surging to 15 percent and poised to rise further.

And with 41 percent of likely caucus-goers saying they could still be persuaded to change their minds, the fight for the largely symbolic crown here could still tip in unpredictable ways.

The survey found 12 percent for former House speaker Newt Gingrich, 11 percent for Texas Governor Rick Perry, and seven percent for Representative Michele Bachmann.

But in the last two days of the four-day poll, conducted this week, Santorum was up to 21 percent, with Paul in third place at 18 percent, while Romney's support remained unchanged at 24 percent.

"Momentum's name is Rick Santorum," said the Register's pollster, J. Ann Selzer, whose survey had a margin of error of plus or minus four percentage points overall and 5.6 percentage points for the last two days alone.

The poll showed how an unrelenting barrage of negative advertisements have hurt Gingrich, who led a similar survey in November with 25 percent.

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 here 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.

Obama's push for a second term has been weighed down by the sour US economy and historically high unemployment, the top issue on Americans' minds nearly four years after he promised his victory would bring "hope and change."

"It's a tough time. I believe this is a detour, however, and not a destiny," Romney said at a rally in the town of Le Mars. "This election is not just an election to replace a president. This is an election to save the soul of America."

"His big asset is his experience in the business sector," Chris Knapp, who backed Romney in his failed 2008 White House run, told AFP in Le Mars. "We need a candidate to defeat Obama, that's the way to go."

Romney still faces stubborn doubts about his conservative credentials and has been unable to increase his support among Republican voters nationwide above 30 percent.

"I'm concerned by his health care plan, it's too similar to Barack Obama's," said undecided voter Michelle Connor, referring to the Democrat's landmark overhaul of US health care, a law Republicans revile.

"But if he picks the right vice president, he's got my vote," she added, saying the former Massachusetts governor could win her over by choosing "somebody socially conservative."

Campaigning in Boone, Iowa, Perry took a veiled shot at Romney, saying voters "don't have to settle" for a candidate who is "trying to tell you that they are conservative but their record is not there."

Perry, who briefly shot to the top of the crowded field only to collapse after a series of poor debate performances, also vowed to "shut down and secure the border with Mexico" against undocumented immigrants.

Paul, whose organized and energetic campaign has him running neck and neck with Romney in Iowa, was in his home state of Texas but due back here on Monday.

Santorum, long seen as a long shot, implored Iowans "don't settle" for a candidate on the basis of their poll strength, "trust your heart, trust your head."

Gingrich also raced across this heartland state, its famed corn fields brown and desolate in the winter chill.

At her campaign headquarters near here, Bachmann -- also a long-shot -- met with cheering volunteers and telephoned Iowa voters.

Romney has hoped that a win here and a victory in New Hampshire could help him seal up the nomination after a few primaries, while former US China envoy Jon Huntsman has skipped Iowa and focused on New Hampshire.