Byron York

Fourth, Michigan seems custom-made for Santorum's message of reviving American manufacturing and paying more attention to the problems of American workers who don't have college degrees. It's something Santorum has been talking about since his earliest days of campaigning in Iowa, and it seems likely to resonate in Michigan.

Fifth, while Arizona is a winner-take-all state when it comes to awarding delegates, Michigan is not. In Michigan, whoever wins a particular congressional district will win that district's delegates. That would allow Santorum to target places where he is strong and have the chance to walk away with delegates even if he doesn't win the whole state. If he doesn't win in Arizona, he gets nothing.

Back in 2008, Romney was the conservative alternative to McCain. This time, Santorum will present himself as the conservative alternative to Romney. "There are still a lot of conservatives in Michigan," says top Santorum strategist John Brabender. "They're conservatives first, and they're not going to vote for Romney just because he has a connection to the state if they don't believe in his principles." Brabender, who says he does not want to tip the campaign's hand on future strategy, just happened to be in Michigan as he was speaking.

Whatever route Santorum picks, the next primaries will test an explanation of his success that first emerged in Iowa. Back then, some Republicans had substantive objections to Romney, or Gingrich, or Rick Perry, or Herman Cain or Michele Bachmann. Those objections were based on policy differences, or "baggage," or just the feeling that the candidate wasn't ready to be president. But when it came to Santorum, most GOP voters worried chiefly that he couldn't win. Once they overcame that hesitation, he won.

Santorum hopes that will happen again. "There's an anybody-but-Mitt crowd," says Chuck Laudner, the conservative Iowa Republican operative who is working on Santorum's behalf. "There's an anybody-but-Newt crowd. There's an anybody-but-Obama crowd. But there's no such thing as an anybody-but-Rick crowd."

Of course, with a lot of campaigning to come, one might develop. That's Romney's job for the next two weeks.

Byron York

Byron York, chief political correspondent for The Washington Examiner