Santorum focused on southern portions of rural Iowa, where the former Pennsylvania senator has made a point of visiting more often than his rivals.
Once overlooked, Santorum has risen in the past several weeks as Gingrich's early December edge evaporated under a barrage of negative advertising by pro-Romney allies. CNN/Time and NBC/Marist polls this past week put Santorum in third place, ahead of Perry, Bachmann and Gingrich for the first time.
Santorum has gained steam as a strong social conservative where rivals Perry and Bachmann have failed to emerge.
Traveling with his daughter Liz, who quit college to campaign for her father, Santorum said he had momentum where other candidates, such as Perry, had money.
"We believe that ultimately, money doesn't matter in Iowa," Santorum said at a packed stop in Indianola. "You can't buy Iowa. You've got to go out and work for Iowa votes."
In a new ad Saturday, Santorum contended he was the field's one true conservative, but also capable of winning, given his election success in Pennsylvania, a swing-state.
The 30-second spot, just the third for Santorum's cash-strapped campaign, called him "a trusted conservative who gives us the best chance to take back America."
Social conservatives who rallied behind Huckabee in 2008 haven't settled on a single candidate this time. For example, Des Moines-area talk radio conservatives Simon Conway and Steve Deace made competing endorsements Friday — Conway for Perry, Deace for Gingrich.
Gingrich has tried to slow his slide by sticking to a positive theme, while continuing to note the beating he's taken by millions of dollars in attack advertising by Paul and a political action committee bankrolled by Romney supporters.
"I think Iowa could actually dramatically change people's understanding of what works in politics if you repudiate that kind of negativity," Gingrich told 150 people at a Council Bluffs restaurant Saturday.
Perry has seen his fortunes improve some, by advertising the most aggressively. Of the more than $3 million in television ads he has spent, the final round included spots promoting him as a Washington outsider, and Santorum, Bachmann, Gingrich and Paul as insiders.
The Texas governor emphasized his Christian faith as he fought for the backing among the influential yet divided, bloc of social conservative voters in Iowa.
Perry's advisers see Santorum within reach and have begun attacking the former senator for having supported spending on home-state pet projects, an unpopular position in these tough economic times.
Santorum charged Perry with hypocrisy: "He had a paid lobbyist in Washington looking for earmarks."
Perry was the first to attack the rising Santorum, who appeals to similar Christian conservatives Perry needs.
Perry was doing his best to visit key areas his later-starting campaign had yet to touch, such as Republican standard Fort Dodge in north central Iowa, as he neared the end of a two-week bus tour.
Despite his effort to portray himself as a reliable conservative, Perry has had to clarify his position on opposing abortion rights.
Perry sought to slow his rival on fiscal, not social issues.
"I think the world of Rick Santorum. He's got a great family. But we've got some real difference when it comes to fiscal issues," Perry told supporters in Boone. "Those differences couldn't be clearer when it comes to important issues in this election like spending."
Meanwhile, Bachmann, who had fallen since last summer to single digits in Iowa polls, was at her Des Moines-area state campaign headquarters rallying supporters and volunteers.
Bachmann was in full mobilization mode Saturday, sticking close by her campaign headquarters to join college students and other volunteers in dialing up potential voters.
Her state chairman, Sen. Brad Zaun, said the campaign isn't paying attention to polling that shows her in last place. He said Bachmann's supporters are deeply passionate about their candidate so are the most likely to turn out — and stay as long as it takes.
At least 10 protesters connected to the nationwide Occupy movement were arrested outside the office in a suburban strip mall.
Bachmann did not come outside of the building.
Associated Press writers Holly Ramer in Campton, N.H., Steve People in Hampton, N.H., and Shannon McCaffrey in Council Bluffs, Iowa, and Brian Bakst in Urbandale, Iowa, contributed to this report.
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