Correction: An earlier version of this story inaccurately reported that Mitt Romney won the Arizona primary in 2008. This version has been updated.

Front-runners Mitt Romney and Rick Santorum opened one of the most crucial periods in the Republican presidential campaign Tuesday, with Romney going on the attack while suddenly claiming the mantle of underdog as he seeks to avoid potentially devastating losses in next Tuesday’s primaries in Arizona and Michigan.

With the candidates scheduled to debate in Mesa, Ariz., on Wednesday night, new polls show Romney gaining ground in Michigan, after Santorum opened up a clear lead there a week ago. The race in Romney’s home state now appears to be a toss-up. Romney and the super PAC supporting his candidacy continue to pound Santorum with negative ads, and both candidates have tried to lower their expectations.

In Arizona, Romney has long been the clear favorite, but his supporters say Santorum has picked up support since his surprise victories in Feb. 7 contests in Colorado, Minnesota and Missouri. A CNN-Time magazine poll released Tuesday showed that Romney’s lead is four percentage points. The fact that voting has been underway all month is expected to boost Romney’s chances.

Romney won Michigan during his 2008 presidential campaign, though he lost Arizona to that state’s Sen. John McCain. Losses on Tuesday, particularly in his home state, would shake the foundation of his campaign and send shudders through a Republican establishment increasingly worried about the party’s prospects of defeating President Obama in November.

Santorum’s surge has raised fresh questions about why Romney has been unable to consolidate GOP support, and it has spurred talk about whether it is too late for another candidate to enter the race. It also has sparked new discussion about whether the race could be decided on the floor of the Republican National Convention in August in Tampa.

But the sudden rise of Santorum, a former senator from Pennsylvania, has brought critical attention to his record. On Tuesday, the Huffington Post reported that his strong antiabortion position was at odds with his earlier views. The Web site found a Philadelphia Magazine article from 1995 in which Santorum said: “I was basically pro-choice all my life, until I ran for Congress.” According to that article, Santorum said he changed his view after reading scientific literature about the practice. Late Tuesday, Santorum adviser John Brabender, after checking with the candidate, said, “He never held a pro-choice position.”

Campaigning in Michigan on Tuesday, Romney hammered Santorum for a second straight day, but not on social issues. Instead, the former governor accused Santorum of being a Washington insider who never left the capital after losing his Senate seat in 2006.

Romney said his rival had supported more spending and debt and voted for numerous earmarks. “I don’t think these are consistent with the principles of conservatism,” Romney said in Shelby Township. “I don’t think Rick Santorum’s record is that of a fiscal conservative.”

Another contender, Rep. Ron Paul (Tex.), piggybacked on Romney’s attacks with a television ad that excoriates Santorum on spending, calling him a “fake” fiscal conservative.

Romney was introduced in Michigan as “the comeback kid” and tried to set himself up as the underdog in a contest in which he has long been considered the candidate to beat. Santorum’s surge has disrupted his campaign and forced him to try to make his candidacy more appealing.

Although Romney focused on economic issues, the question-and-answer period centered on social concerns. One voter asked Romney to confirm that he opposes abortion. Another asked about a recent Obama administration decision on religious institutions and contraception coverage.

“I can assure you, as someone who has understood very personally the significance of religious tolerance and religious freedom . . . I will make sure we never again attack religious liberty in the United States of America,” Romney said.

Campaigning in Phoenix, Santorum never mentioned Romney by name during a speech at the Maricopa County Lincoln Day lunch. But he challenged any suggestion that he was a big-spending Washington insider who followed the party establishment.

As a new House member and a new senator, he said, he fought for reforms that shook the institutions to their core. Addressing accusations that he wasn’t a true fiscal conservative, he said he never supported a tax increase. “I voted for smaller government, lower taxes, less regulation.”

Santorum spent little time addressing economic issues and concentrated instead on the threats to liberty and freedom he said the current administration poses. He said he is under attack in the media, as other Republican candidates have been, and brought the cheering audience to its feet, saying: “Will you sit on the sidelines and say, ‘Boy, that’s not fair,’ or will you stand up and fight back for freedom?”

He closed with comparisons between himself, Romney and former House speaker Newt Gingrich (Ga.), another rival for the GOP nomination. “I’m not a manager. I’m not a visionary,” he said. “I’m a guy from a steel town who grew up understanding what made this country great.”

Wednesday’s debate, which airs on CNN at 8 p.m. Eastern time, could play a crucial role in shaping voters’ views of the candidates. Santorum, who holds a 10-point lead in Gallup tracking of the Republican race, will be in the spotlight after a series of weekend comments that were made about Obama and issues that caused new controversy. But Romney, too, will need to reassert himself and try to convince those in the party who question his conservative convictions.

The forum will mark the first time the four remaining candidates have shared a stage since Romney’s victory in the Florida primary on Jan. 31. Then, the race appeared to be between Romney and Gingrich, and the former Massachusetts governor’s win in the Sunshine State seemed to put him in command of the nomination contest. That was reinforced by his victory a few days later in the Nevada caucuses. But Santorum’s unexpected wins on Feb. 7 shook up the contest once again.

Randy Pullen, a former Arizona GOP chairman and a Romney supporter, voiced the nervousness within the former governor’s camp Tuesday at the luncheon where Santorum spoke. He said he thought Romney was ahead in Arizona among those who have voted early, but, he added: “I’m concerned. If there’s a big turnout on election day, Santorum might pull it off.”

Somashekhar reported from Michigan. Staff writer Felicia Sonmez contributed to this report.