Already, Rick Santorum has surged ahead in polls in Michigan, suggesting that Mr. Romney's home-state advantage (he was born and raised there, and his father was a Michigan governor) won't mean much.
But what about Arizona?
The state, with a prize of 29 delegates, looks much better for Romney. It's in the West, where he tends to do better, and it has a sizeable Mormon population – about a tenth of the Republican electorate. The latest state poll, by Rasmussen, has Romney leading the field by 24 points. Mr. Santorum isn't even in second place (that spot belongs to Newt Gingrich), but is lagging in third with 13 percent.
Keep in mind, though, that the poll was taken on Feb. 1 - a week before Santorum's impressive three-state victory in Minnesota, Colorado, and Missouri, which gave him an enormous boost around the country.
At this point, a Romney victory in Arizona still seems highly likely, but the state will provide an interesting counterpoint to Michigan in two weeks. In particular, it will be a testing ground for whether Santorum's social-conservative message can resonate in a less traditionally religious state.
Unlike voters in some of the Santorum-friendly Midwest states, Arizona Republicans tend to be a bit more Libertarian. Santorum may find more fertile ground if he tries to drum up populist support and anger – this is a state hurt hard by the housing crash, for instance – by emphasizing, say, Romney's Wall Street ties, rather than his lack of conservative credentials.
Both candidates may also have to take on immigration issues in a much more direct way than they have to date. (Look for this to be a big topic at the only debate between now and the primaries, held in Arizona on Feb. 22.)
For now, Santorum is focusing most of his energy and money on Michigan, which he has a far better chance of winning. But if he can put in a strong second-place showing in Arizona, even if he doesn't win, it could still be a boost for his candidacy.
Romney spent Monday in Arizona, before heading to Michigan later this week, and sounded his new theme: making the case that he is a true conservative.
“My conservatism did not come so much from reading the writings of great conservative scholars as it did come from my living my life, my family, my faith, my business,” he told the crowd in Mesa, Ariz.
He also tried to undercut Santorum, without mentioning him by name, by emphasizing the importance of experience outside of government.
“People go [to Washington] and they get infected by this Washington disease which makes them think that government is the source of our greatness, that government should guide our lives and they become more and more insistent on intruding government into our lives," he said.
Early voting is already underway in Arizona. And Romney is likely hoping to convince as many voters as possible to register their support for him now, before Santorum has a chance to gain a foothold in yet another state.