Once thought to be an easy win for native son Mitt Romney, the Michigan primary has instead turned into one of the stiffest tests for the Republican presidential front-runner — and an unexpected opportunity for the surging campaign of Rick Santorum.

After his surprising sweep of three states last week, Santorum is targeting the Feb. 28 contest, seeing in Michigan the combination of working-class and socially conservative voters on which he has based his campaign. And in Michigan, the former senator from Pennsylvania looks like he will get the one-on-one match-up with Romney that he has been craving — as long as Newt Gingrich and Ron Paul continue to largely look past the state.

The Romney and Santorum campaigns began their battle in earnest on Tuesday, with both beginning new ad campaigns — Romney playing up his Michigan roots and Santorum arguing that he would be a more clear contrast to President Obama in November.

Santorum, who has moved into a virtual tie with Romney in polling nationwide, including in Michigan, plans to give a major economic address before the Detroit Economic Club on Thursday. Romney will deliver his own economic speech to the group next week.

Romney wrote an op-ed piece in the Detroit News on Tuesday to try to explain his opposition to the 2009 automobile industry bailout, which Democrats argue has been key to turning around General Motors and Chrysler. In the piece, Romney wrote that Obama mismanaged the bailout by giving unions too much power.

“It’s going to be a slugfest here. We didn’t think we’d matter, and now we’re at the eye of the storm,” said James Muffet, president of Citizens for Traditional Values, a Michigan-based group that has an e-mail list of 15,000 social conservatives and has not endorsed a candidate in the race.

There are potentially huge opportunities — and pitfalls — in Michigan for Romney and Santorum.

For Romney, the contest is a chance to show that he can connect with working-class voters, who have been lukewarm toward him. He has a built-in advantage in a state where he grew up and his father was a popular auto executive and governor. But for those same reasons, a defeat here could be devastating.

“If Romney loses Michigan, the perception is that it’s just a huge loss for him, one that could really cost him the nomination,” said Steve Mitchell, a Republican pollster based in East Lansing.

For Santorum, the Michigan contest is a chance to show that he can appeal to more than social conservatives and that he can mount the kind of broad-based campaign needed to win the nomination — and defeat Obama in the fall.

While less damaging than for Romney, a loss for Santorum could sap his momentum as the campaigns head into 10 contests on Super Tuesday just a week later. It also could raise questions about Santorum’s ability to defeat Romney in races where the front-runner exerts real effort.

Both camps have been working to play down expectations. In a Fox Business Network interview last week, Romney said he didn’t expect Michigan “to be a landslide.” And Santorum adviser Hogan Gidley said Tuesday that the campaign is “under no illusions” that a Michigan win is likely.

In the 2008 Republican presidential primaries, Romney beat the eventual nominee, Sen. John McCain (Ariz.), by nine percentage points here. But in that race, Romney ran as the conservative alternative to McCain, a role Santorum is playing this year.

Still, Romney has a home-field advantage: He grew up in Bloomfield Hills, outside Detroit, and met and wooed his wife in the state. The Romney family has vacationed in Michigan for years, and the state is in some ways more of a home base for the Romneys than Massachusetts.

“In Michigan, we’ve always felt like the rest of the country either doesn’t understand us or wishes we weren’t here,” said Rep. Thad McCotter, one of seven Republican members of Congress from Michigan who have endorsed Romney. “It’s nice to know that someone’s running for president who knows the state and is from here.”

Rep. Bill Huizenga (Mich.), another Romney supporter, recalled attending his first Republican state convention at age 18 in 1988. Although he has been out of office for 25 years, George Romney, Mitt’s father, received the convention’s most enthusiastic ovation, he said.

“There’s still a lot of goodwill for the Romney family in Michigan,” Huizenga said.

Muffet said Romney has an impressive organizational network that has been visible to grass-roots activists in the state for years. And an independent group working on Romney’s behalf announced Monday that it had purchased $600,000 in ad time to blanket Michigan airwaves in the next week.

Romney hopes to use his resource advantage over Santorum to press the message that the former senator is a Washington insider who didn’t do enough to cut spending in his time in Congress and sought now-hated earmarks for his state.

Romney’s campaign also thinks that his business background and experience turning around troubled companies will play well here.

“The guy knows Michigan and he knows manufacturing,” Huizenga said. “It’d be a huge embarrassment if he’s not able to win the Michigan primary. But I believe he can and will win it.”

One Romney adviser acknowledged that the campaign has been surprised by Santorum’s rise in Michigan polls and acknowledged that the idea that he could be a real threat is “troubling,” given that Michigan is an important swing state with 30 delegates at stake.

Even a strong second-place finish could benefit Santorum, if it helped establish him as Romney’s chief competitor. Additionally, Michigan’s proportional delegate allocation rules mean that he could nearly match Romney’s delegate haul in the state by running a close second.

Santorum supporters think Romney is vulnerable in Michigan and have made it clear that they will contest the state more vigorously than Arizona, which will hold its primary on the same day.

“We think Michigan is a great place for us to plant our flag and talk about jobs and manufacturing and giving opportunities for everybody in America to rise,” Santorum said on MSNBC’s “Morning Joe” last week.

Supporters think Santorum, whose grandfather was a coal miner and who has stressed a plan to revive manufacturing by eliminating corporate taxes for industry, speaks better to Michigan’s blue-collar Reagan Democrats than Romney does. They also say Catholics and evangelical voters, an important part of the Michigan base, are rallying around Santorum.

“We think Rick’s comprehensive conservative message really resonates well,” Gidley said. “Rick is the kind of guy who can relate to the struggles of the Michigan voter.”

Stuart Roy, a spokesman for the Red, White and Blue fund, an independent super PAC working on Santorum’s behalf, said his group is polling in Michigan and elsewhere and will be making investment decisions in the coming days.

“Part of this is science and part of this is gut — but, at this point, it’s figuring out how well you think Gingrich is going to do, as much as anything else,” he said.

Gingrich had said weeks ago that he thought Romney was vulnerable in Michigan and he threatened to contest the state. But spokesman R.C. Hammond said Monday that the campaign has no staff on the ground. With limited resources, he said the team is focusing on states where it can match Romney “dollar for dollar” and win, citing the Super Tuesday states of Georgia, Ohio, Oklahoma and Tennessee as examples.

Hammond said Gingrich realizes that contesting Michigan now would probably only peel votes from Santorum, strengthening Romney. Instead, Gingrich said that Romney would be hobbled by a Michigan loss to Santorum — an outcome that would create more opportunities for Gingrich.

“The goal right now is to get Mitt Romney out of the race,” he said.

Staff writers Philip Rucker in Washington and Amy Gardner in San Diego contributed to this report.