Two days before the voting begins in the wildest Republican race anyone can remember, the GOP candidates for president were engaged in a frenzy of old-school retail politicking acutely aware that a poor finish in Tuesday’s Iowa caucuses would probably end some of their prospects.

On Saturday evening, the Des Moines Register released a poll showing a highly volatile race, with former Massachusetts governor Mitt Romney holding a slight lead at 24 percent among likely caucus attendees and Rep. Ron Paul (Tex.) in second with 22 percent. But former Pennsylvania senator Rick Santorum, showing a late burst of momentum that has brought him from the back of the pack to 15 percent, was poised to move into second place if he can continue gaining over the next two days.

Meanwhile, three former front-runners were struggling to regain their footing, with former House speaker Newt Gingrich at 12 percent, Texas Gov. Rick Perry at 11 percent and Rep. Michele Bachmann (Minn.) at 7 percent.

Caucuses are notoriously difficult to predict, given the fact that they require voters to venture out on a winter night and spend an evening arguing politics with their neighbors, but the Register’s late poll has had a strong record of foreshadowing the results.

In recent days, the candidates’ arguments have pitted voters’ pragmatism against their passions, with Romney representing the safe, establishment-approved pick and his rivals vying to be the conservative alternative.

“This is a process not just of putting your name or your hand next to someone who you kind of like. It’s also selecting who our nominee ought to be, who you think could beat Barack Obama,” Romney told a crowd of hundreds Thursday afternoon as he stood on a chair in the faux “Music Man” set in Mason City.

But on Saturday, as Santorum addressed about 50 people outside a library in Indianola, he insisted: “I understand they’re all saying who can win and cannot. Trust your own heart. Trust your head. Trust your gut. And vote for who you think is best.”

Among the serious caucus contenders, only Paul was missing. He and his senator son, Rand Paul of Kentucky, will be back Monday to launch a five-county tour.

Ron Paul has lately found himself at the top of polls, joining a procession of contenders — some credible, others less so — who have soared and fallen, often within a matter of weeks.

Some have stumbled, spectacularly. Others have been pushed. Gingrich was hit by almost $3 million in negative advertising in Iowa from a Romney-aligned super PAC — an outside group barred from coordinating with his campaign.

The volatility reflects Republicans’ fervor to pick their strongest nominee against a vulnerable president and the dissatisfaction and mistrust many conservatives, especially those who align with the tea party movement, feel toward Romney.

Ralph Davey, 60, a retiree from Manly who came out to hear Gingrich speak at the local shopping mall last week, has been going back and forth over whether to support him or Romney.

Romney “knows a lot about business, and he’d be able to create jobs. But he seems wishy-washy on issues. He seems to change with the circumstances,” Davey said. “I like his electability. He doesn’t have skeletons in the closet. Newt definitely does.”

Romney’s late push

Paradoxically, the tumult in the field and the fragmentation of the electorate may have worked to Romney’s benefit, creating an opportunity for him to prevail in a state where he was trounced four years ago.

It was once expected that Romney would put forth no more than a token effort in Iowa, where he blew $10 million on a distant second-place finish in 2008. But as it became clear that a strong showing — even a victory — might be possible, Romney has put in a heavy campaign schedule here in the final days.

Iowa has a history of knocking establishment GOP candidates down a peg, so if Romney can avoid that jinx he would be well situated going forward.

Romney is the overwhelming favorite in New Hampshire, which holds its primary Jan. 10. And it is questionable how strong a threat Paul or Santorum would be over the long run. On the other hand, if Gingrich or even Perry were to get a bump out of Iowa, they could potentially pick up momentum.

Romney originally planned to spend more time over the weekend in New Hampshire. But he was drawing large and enthusiastic crowds here, and Romney’s aides scratched that plan so he could dart back to Iowa. He added two afternoon campaign stops in the heavily Republican northwestern corner of the state, where he performed well in 2008, and he has scheduled four large rallies Monday in the state’s population centers.

This campaign season has also, again, proved that experience is an asset when it comes to running for president. It is probably not a coincidence that two of the candidates best positioned for Tuesday’s caucuses — Romney and Paul — are the ones who have been there before.

“People like Ron Paul and Mitt Romney are benefiting from the fact that they had infrastructure from the past that they could build on,” said Steve Scheffler, president of the Iowa Faith and Freedom Coalition.

Many political veterans here expect turnout at the caucuses to top 2008’s record 118,000. But they worry that Iowa’s cherished, quirky process has begun to lose its intimate feel — and its influence.

Iowans have seen far less of the candidates this year than they have in the past. The GOP contenders started their campaign operations here later and invested less of their time and resources. Instead of getting to know potential presidents in a leisurely fashion at their local diners and in church halls, Iowans are learning about the presidential field on television, just like the rest of America.

That has contributed to voters’ uncertainty, Scheffler said.

“The candidates haven’t been here, and their messages and their stands are similar,” he said. “You don’t have that one personality that sticks out from the others.”

Adam Gregg, 28, a Des Moines lawyer, came to see Romney speak at an ice-cream parlor in Le Mars on Saturday.

“Back in ’06 and ’07,” Gregg said, “you saw all the presidentials going to state legislative fundraisers. They were all over the Iowa circuit. That’s one of the starkest contrasts to me.”

Santorum is one who has been doing things the old-fashioned way — more by necessity, because of his strapped finances, than by choice.

Having made more than 250 appearances in the state over the past six months, “a guy like Santorum gave Iowans all across the state multiple opportunities to meet him and get to know him,” said Craig Robinson, a blogger who writes for the Web site the Iowa Republican.

That patient, shoe-leather approach is paying off with an eleventh-hour surge in the polls for Santorum, who noted with some satisfaction that he has not had to spend the final days before the caucuses crisscrossing Iowa on frantic bus tours, as his rivals have.

“Everybody’s sort of running around trying to get to all these counties. We’ve done all that,” he said during a leisurely interview Friday afternoon at his campaign headquarters in Urbandale.

Later that evening, Santorum spent more than an hour at a sports bar, joining fans to watch Iowa State play Rutgers in the New Era Pinstripe Bowl.

Evangelical vote split

One significant difference from 2008 is the lack of political cohesion among evangelicals, whose mobilization was the key to former Arkansas governor Mike Huckabee’s victory over Romney in 2008. This time around, however, there are at least four candidates competing for their votes.

“No one candidate has brought all the values voters together,” Huckabee said in an interview. “And the splitting of that vote helps Romney.”

The political climate may also diminish the influence of socially conservative voters here.

“Economic issues are at the forefront now, even among conservatives, unlike they were four years ago,” said Tim Albrecht, who worked for Romney’s operation in 2008 but now is unaffiliated and is an aide to Iowa Gov. Terry Branstad.

The Iowa caucuses have provided the first real test of the new world of outside money in presidential campaigns, unleashed by a Supreme Court decision last year.

Spending by outside groups — technically independent of the campaigns but often run by some of the candidates’ closest allies — used to be virtually nonexistent in primary campaigns. This year, it has accounted for 43 percent of all spending on television ads.

The tone of the ads in many cases has been far more negative than the candidates personally would dare to project when speaking about members of their own party.

Almost none of the donors of that money have been revealed, and they probably won’t be until Jan. 31 — after the polls close in the Florida primary and after a nominee may already be chosen.

The new rules have given a boost to candidates who already have a network of rich supporters — most notably Perry and Romney. This kind of support may soon be an unwritten requirement for a successful campaign.

Gingrich initially believed that he could overcome the influence of negative advertising with a strong message and the kind of coverage he draws.

“That’s the secret of the Gingrich campaign,” he said in an interview in late October. “The other guy gets to raise a lot of money to buy radio ads, and I get to do talk radio. It’s probably almost impossible to buy enough radio to offset what I can do with talk radio.”

The most recent polls suggest that view may have been overly complacent.

At one point Gingrich said, “I wouldn’t vote for the guy they are describing.”

Staff writers T.W. Farnam in Washington and Amy Gardner, Philip Rucker and Rosalind S. Helderman in Iowa contributed to this report.