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 responding by attacking his opponent'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.
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.
"I grew up with the Romney name. My parents were big Romney fans," said Ken Leonardi, owner of a restaurant in Mount Clemens, where Romney greeted several dozen supporters.
The restaurant, The Mitt, wasn't named for Romney but for the mitten shape of Michigan, although Leonardi is a Romney supporter.
"I'm planning on winning, by the way," Romney said.
Even if Santorum beats Romney, Romney will emerge with a healthy share of delegates to add to his lead in the count, never mind fundraising and campaign organization in a race that was designed to carry into the spring.
There is the chance for a split decision if Romney wins the popular vote but Santorum comes away with more delegates. Most of the state's 30 delegates are awarded two at a time to the winner in each of the state's 14 congressional districts.
Still, Romney is far better organized in states that are voting March 6, such as Ohio and Southern bastions Georgia and Tennessee, once thought to be safe for Gingrich.
Santorum could be in for a shock if he loses Michigan, much like Gingrich did after shooting to the top of the polls in Florida following his South Carolina primary. He then lost Florida and struggled at fundraising afterward.
Michigan has a reputation for at times bucking the national trend, including Romney's victory over the eventual nominee, John McCain, four years ago.