Mitt Romney is in trouble in Michigan. What happens if he loses there?

It’s now a distinct possibility that Rick Santorum could win in the state where Romney’s father was governor. Nate Silver’s prediction model, which only looks at polling, actually gives Rick Santorum a 77% chance; the four public polls released in the last week have Santorum with leads ranging from 3 to 16 points. That almost certainly understates Romney’s chances. Some of the lead likely reflects a bounce from Santorum’s recent wins that could easily dissipate, and Romney still holds a huge advantage in resources. Still, there’s clearly an opening for Santorum.

One consequence of a Romney loss in a state where he has deep ties is that it could help Santorum on Super Tuesday, March 6th. Not only could it boost him in the key Ohio battleground, but it could mean further shifts of voters in southern states from Newt Gingrich to Santorum. And that’s what Santorum would need if he is really to be a serious contender for the nomination: wins in the South, probably starting with Oklahoma, Tennessee, and perhaps even Georgia. That’s unlikely, but it’s at least plausible if Santorum wins in Michigan on February 28.

Perhaps more important, a Michigan win for Santorum will guarantee that the seriously contested portion of the nomination fight continues beyond Super Tuesday. Even if Romney does very well on that day, Santorum will have done well enough in the previous month that the bulk of the press will treat a Super Tuesday win as merely a good showing for Romney, and will not declare that he has wrapped up the nomination. This means he’ll still have to show up at debates, run primary-oriented ads, and otherwise continue to delay his shift to a full assault on Barack Obama.

Even if all of this happens, Romney would still be the favorite to win the nomination. And my view is still that he’s the favorite to win Michigan, which would leave him at least with a possibility of ending it all on March 6th. But without Michigan, that just won’t happen.

Analysts disagree about how much long, divisive nomination contests matter for the fall election. But you can be sure the Romney and Obama campaigns believe it would be a significant disadvantage for Romney to have to spend all of March (and April? And May?) worrying about the votes of GOP primary voters who place a priority on blocking access to birth control coverage and ending Medicare as we know it, rather than trying to win over general election swing voters who care mostly about jobs and the economy.

So while it’s an exaggeration to say that Michigan is make-or-break, it’s critical to how the nomination process plays out, and perhaps even to what happens in November.