(AP Photo/Paul Sancya)

COLUMBUS, Ohio - Rick Santorum is raising the stakes in his battle with Mitt Romney for the Republican presidential nomination by waging an aggressive campaign in the U.S. Midwest industrial heartland.

Santorum is leading in polls of Republican voters in Michigan, Romney's home state which holds its presidential primary on Feb. 28. He is also leading in polls in Ohio, where he declared the state "ground zero" for his campaign on March 6, or Super Tuesday, when 10 states hold nominating contests to choose a candidate to face President Barach Obama in the November election.

Santorum, a former Pennsylvania senator, has positioned himself as the leading conservative in the race, while Romney, a former Massachusetts governor, has struggled to win over the party's conservative base and has been attacked for his shifting positions on sensitive issues like abortion and health care reform.

Romney not only grew up in Michigan, he is the son of a former governor. The primaries in Michigan and Arizona on Feb. 28 mark the end of a lull in the state-by-state contests to choose delegates to the party's national convention in late August in Tampa, Florida.

Though Santorum and Romney are close in the polls, Romney is considered the Republican front-runner nationwide, with more delegates from state-by-state voting that are necessary to claim the party's nomination. The multimillionaire also has far more money and a much stronger campaign organization than Santorum.

The two other rivals for the Republican Party nomination are former speaker of the U.S. House of Representatives Newt Gingrich and the libertarian-leaning Texas Rep. Ron Paul.

After three events in Ohio on Friday, Santorum also planned to spend Saturday campaigning in the state, where he is actively courting small government, anti-tax tea party activists and evangelical voters.

Santorum, who is known for his social conservative policies, questioned Obama's Christian values at a tea party rally in Columbus, Ohio. He said the president's agenda is "not about you. It's not about your quality of life. It's not about your jobs. It's about some phoney ideal. Some phoney theology. Not a theology based on the Bible. A different theology."

More delegates will be awarded in Ohio than in any other state except Georgia in the opening months of the Republican campaign. Ohio and Georgia are two of the 10 contests scheduled for March 6, a benchmark for the primary campaign that often decides who can continue to the next level.

"There's no state that can shout louder. You are the biggest state. You've got the biggest trove of delegates," Santorum told the Brown County Republican Party on Friday night. "This is ground zero. Ohio."

While 63 delegates are at stake in Ohio, Georgia offers 76.

Hours earlier, the former Pennsylvania senator stood at the State House in Columbus as state Attorney General Mike DeWine formally shifted his allegiance to Santorum from rival Romney, another sign that Santorum has seized the momentum in the roller coaster Republican presidential contest.

His socially conservative message has captivated crowds this week from Boise, Idaho, to Romney's hometown of Detroit to the southern Ohio village of Georgetown.

"We have a culture that is in need of renewal," Santorum declared inside the Georgetown Elementary School gymnasium. "Big things are at stake. Our family. Our faith."

As the Republican candidates were distracted by their nominating fight, Obama has seen is ratings in the polls steadily rising as the poor economy - by far the most important issue in this presidential race and his biggest weakness - continues to show signs of a recovery. He also scored a victory this week over congressional Republicans by gaining approval of a bill to extend a payroll tax cut.

Obama, who was concluding a West Coast fundraising trip, raised $29.1 million for his campaign in January, a strong month that added to his momentum in the re-election campaign.

The month's haul raises Obama's total combined fundraising for this election cycle to about $250 million.

Raising money is even more important in this election after a Supreme Court ruling opened the door to big-money donations to groups that support campaigns. The so-called super political action committees are supposed to be independent of the campaigns, but critics have pointed out the possibility for abuse.

Questions about whether Santorum can sustain his rise in the polls come amid signs of stress within his campaign, mainly disorganization. Romney's mammoth political machine - coupled with new scrutiny for Santorum's view of social issues as well as governmental policies - will give Santorum little margin for error.

As an example, one misstep by a Santorum supporter kept the former senator off message at times for two days.

Foster Friess, the main donor behind a super political action committee supporting Santorum, created a stir Thursday when he related on MSNBC an old joke about how aspirin used to be a method for birth control. "Back in my days, they used Bayer aspirin for contraception," Friess said with a grin. "The gals put it between their knees and it wasn't that costly."

Friess apologized Friday in a blog post. But Santorum was repeatedly forced to distance himself from his surrogate's comments, which Santorum described as "a bad joke." The comments drew unwanted attention to Santorum's own musings about contraception and women's issues.

Santorum has said that he wouldn't try to take away the birth control pill or condoms but that states should be free to ban them. He told a Christian blog last year that as president he would warn the nation about "the dangers of contraception" and the permissive culture spit encourages. He's also questioned whether women should be in combat and said that "radical feminists" have undermined the traditional family by "convincing women that professional accomplishments are the key to happiness."

The contraception flap, according to Republican observers, is evidence of an undisciplined campaign that is already stumbling under the weight of intensifying scrutiny. Polling suggests that significant numbers of voters still don't know Santorum well. And he may struggle to win over female voters in particular as they begin to pay more attention, according to Phil Musser, a Republican strategist who doesn't work for either campaign.

"I think in the next couple days, we could start to see some serious erosion with respect for female support for Santorum in the Republican primary," said Musser. He said Santorum could face an even bigger problem with women in the general election campaign against Obama.

The Romney campaign countered on another front in a conference call at roughly the same time as Santorum's DeWine announcement.

John Sununu, a former White House chief of staff and a Romney supporter based in New Hampshire, described Santorum as "a candidate who loves spending and frankly supports liberal labour causes and liberal social causes, like giving voting rights to felons."

Santorum, while in the Senate, supported restoring voting rights to felons once they had completed their sentence or parole.