LANSING, Mich. - Losing the Michigan primary would strip the last of the varnish off the image of Mitt Romney as the inevitable GOP presidential nominee and commit him to the long march he says he's in shape for.

A victory Tuesday by rival Rick Santorum on Tuesday would be a public-relations nightmare for Romney, who was born and raised in Michigan. But Romney's campaign still would carry on with more money than any candidate and remain better organized to compete to the end.

"There is no doubt that if he loses Michigan, perception-wise, the wheels come off the wagon," said Greg McNeely, a former Michigan Republican Party director who's now unaffiliated with any White House campaign. "Can he come back? Absolutely. But it destroys the inevitability perception that has been built around the campaign."

Santorum has shot up in state polls and even leads Romney in some. The former Pennsylvania senator is riding a wave of momentum after winning Feb. 7 caucuses in Minnesota, Colorado and a nonbinding primary in Missouri.

He told tea party members in St. Clair Shores on Saturday that the race in Michigan was close and "winnable," and dismissed as "laughable" Romney's claims that Santorum wasn't conservative.

Romney is trying to attack Santorum's credibility, a strategy used successfully against Newt Gingrich in Iowa and Florida to stop the former House speaker's momentum after winning in South Carolina.

During a breakfast stop in Lansing, Romney said this year's presidential vote was "an election about the soul of America." He stuck to challenging Santorum's commitment to conservative principles in light of votes that Santorum has said he now regrets.

At suburban Detroit restaurant on Friday, Romney listed positions Santorum said he had taken to support his party even though they defied his principles. Such attacks underscore the urgency of Romney's effort to blunt Santorum's challenge, as do the heavy concentration of television ads Romney is airing and the three campaign events he planned in the state Saturday.

Yet, there's a lighter touch at times.

Romney reminded an audience of business leaders of his boyhood in Michigan, tossing out the name of the Detroit hospital where he was born and the school where he attended kindergarten. One television ad features his father, a former Michigan governor and car executive.

Even more, he offered a prescription for the long-term revival of the all-important automotive industry, calling for union concessions and less aggressive emissions standards. Those proposals might help soften any hard feelings caused by Romney's opposition to the federal bailout of the auto industry.