DES MOINES, Iowa- Mitt Romney held a slim lead Sunday atop the field of Republican presidential hopefuls in Iowa, going into a two-day campaign blitz before the state casts the first votes of the US election season. But with four in ten Iowans telling pollsters they could still change their minds, veteran Texas Representative Ron Paul stood within striking distance of the former Massachusetts governor and millionaire financier. And firebrand social conservative Rick Santorum's support was surging among Republicans expected to turn out Tuesday when Iowa holds its caucuses.

 and weighs in on who should face off against President Barack Obama in November elections.

"I may come in first, I may come in second. I doubt I'll come in third or fourth," Paul, known for anti-interventionist and libertarian views that have drawn heavy fire from his rivals for the party's nomination, told CNN.

And Paul denounced as "propaganda" his rivals' increasingly fierce attacks on his unorthodox positions, insisting to CNN that he is "pretty mainstream" and saying his critics "are the ones who can't defend their records."

The Des Moines Register newspaper's final poll before the caucus found Romney with 24 percent support, Paul at 22 percent, Santorum at 15 but rising, and 41 percent of likely voters saying they could still change their minds.

"It's a wide open race," Iowa Governor Terry Branstad, a Republican, told CNN, adding that many voters were still hunting for "the perfect candidate."

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.

"I think Tuesday night people are going to see a miracle," Bachmann, whose long-shot hopes rest heavily on Iowa. "People make their decision, quite honestly, in the caucus room."

Iowans 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.

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.

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.

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's January 10 primary.

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 Saturday in the town of Le Mars. "This is an election to save the soul of America."

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.

The candidates were slated to hold nine campaign events on New Year's Day and nearly 30 total before Tuesday's ballot.

Obama, who is running unopposed, planned to make remarks to be beamed to Democrats in Iowa, which propelled his historic 2008 victory but is expected to be far less welcoming in 2012.