Let's look at both potential scenarios.
ROMNEY WINS MICHIGAN: (...) A loss in Michigan would effectively end Santorum's moment, at least for now, and once again establish Romney as the frontrunner for the nomination.
Other than Romney, the candidate who has the most to gain from a Santorum loss in Michigan is Newt Gingrich. Both Santorum and Gingrich are hoping to do well in the Southern states that begin voting on March 6, when ten states hold contests on what has become known as Super Tuesday. A Santorum loss in Michigan opens the door for the former House speaker to reassert himself as a plausible conservative alternative to Romney, while a Santorum win may close the Gingrich window for good.
Romney winning Michigan (and, presumably, Arizona) doesn't hand the former Massachusetts governor the nomination. Perhaps the most important Super Tuesday vote is in Ohio, where a new Quinnipiac University poll shows Santorum leading Romney 36 percent to 29 percent; Romney would likely get a boost out of a Michigan win, but Santorum - who hails from nearby Pennsylvania - won't go away without a fight. And no matter what happens, Romney is expected to have a hard time in at least three other Super Tuesday states: Georgia (Gingrich's home state), Tennessee and Oklahoma, deeply conservative Southern states relatively hostile to the wealthy, Ivy league-educated Romney. A new Vanderbilt University poll in Tennessee shows Santorum leading Romney there 38 percent to 20 percent.
The good news for Romney is that he still has more money, better organization and more establishment support than any of his rivals. And if he wins both states Tuesday, he'll be able to claim momentum as well.
SANTORUM WINS MICHIGAN: And the Romney campaign goes into full panic mode. There will be headlines about how Romney can't win in the Midwest, can't win conservatives, and can't win working-class voters; his recent spate of tone-deaf comments inadvertently spotlighting his wealth (most recently that he doesn't really follow NASCAR but hey, he's got friends who own teams) will be spotlighted to argue that Romney's claim to be the most electable candidate against President Obama simply doesn't hold up. Romney won't be finished - see the advantages mentioned above, along with the fact that only he and Paul are on the ballot in Virginia, which votes on Super Tuesday - but he will be reeling, and he'll need to find a way to win in Ohio to keep the wheels from going off completely.
Santorum, meanwhile, will be able to claim frontrunner status, and will likely build on his Michigan win with victories in Tennessee, Oklahoma and maybe even Georgia -- which would effectively knock Gingrich out of the race. The contest would then likely become a long slog that could last all the way until June, with Romney, Santorum and Paul battling for delegates and tearing each other down as President Obama looks on happily from the sidelines.
The situation would also increase the prospects of a contested convention, in which no candidate has the 1,144 delegates necessary to secure the nomination outright. That's good news for Paul, a long shot for the nomination who has focused on accruing delegates and could play kingmaker at a contested convention. In exchange for his delegates, Paul could force the nominee to adopt changes in the party platform that move the GOP closer to Paul's brand of small-government libertarianism.
It is not just the Romney campaign who would go into panic mode following a Santorum win. Many Republican party elders would be grasping for the proverbial red button. There are deep concerns among establishment types that Santorum, with his unapologetic embrace of social conservatism, would both lose badly in November -- and be a drag on Republicans in their effort to hang onto the House and take over the Senate.
If Santorum wins Tuesday, expect to see the calls for a last-minute candidate such as Jeb Bush, Mitch Daniels or Chris Christie to come in at the last minute and try to keep Santorum from the nomination. It's not clear how that's possible - most of the primary filing deadlines have passed, and even in the case of a contested convention, Santorum won't simply stand aside. but that won't keep a good chunk of worried establishment Republicans from trying to push him.