Tuesday's votes in the Republican presidential primaries in Michigan and Arizona are the biggest tests Mitt Romney's faced since all the other big tests he's faced, but no matter the outcome, the narratives of what happens next have already been written. If he wins Michigan, he proves he can win over blue-collar voters and conservatives, if he loses, he'll have lost, despite all his money and endorsements, to Rick Santorum, the guy who's talking about "the sexual realm" and how John F. Kennedy's speech on the separation of church and state makes him want to "throw up." Here's our guide to what happens if either candidate wins Michigan and Arizona.

Outcome: Romney wins both Arizona and Michigan.
Likelihood: Good. Though Romney and Santorum are polling really close in Michigan, Romney has been improving in the last few days and Santorum hasn't. Romney's polling way ahead in Arizona. 
What happens: Pundits declare the race over, just as they did after New Hampshire and Florida. Michigan, in particular, is being called a "crucial test" or a "testing ground" for Romney, his opportunity to prove "has the best chance of beating President Barack Obama," The Wall Street Journal writes, as well as that he "can appeal to the party's core social conservatives and tea-party supporters who have resisted him in prior contests." But of course, Romney has already faced such crucial tests. In Florida, where he won by a large margin despite being beaten by Newt Gingrich 10 days earlier in South Carolina, Romney "passed a crucial test," he "proved he could take a punch and bounce back," and the vote "set the front-runner back on a glide path to the 2012 presidential nomination." So please use caution when assuming the primary's done. The candidates will go on to Super Tuesday. Many of the 10 states voting that are less friendly to Romney, but a double victory might convince some voters that the primary is over. With several southern states voting, March 6 will be Gingrich's chance to be a triple Lazarus.
Chance the primary will really, really be overLow.

Outcome: Santorum wins Michigan, Romney wins Arizona.
Likelihood: Moderate. Public Policy Polling shows Santorum regaining some ground Monday night. But that's just one poll.
What happens: "If Mr. Romney has trouble with blue-collar voters, doubts will deepen over whether he has the stuff in a one-on-one race in November to appeal to working-class voters in the states from Pennsylvania to Wisconsin," The Journal reports. A political analyst put it to ABC News' Michael Falcone more succinctly: for Romney, it will be "Chernobyl." It will be particularly embarrassing to Romney, who grew up in the state and is running against a guy the Washington Examiner's Byron York marvels is still "winging it." Still, just as no one is expecting Santorum (or Gingrich) to bow out if Romney wins big, no one's expecting Romney to roll over if Michigan goes against him.
Chance the primary will really, really be over: Very low.

Outcome: Santorum wins both Michigan and Arizona.
Likelihood: Low. Santorum is averaging 16 points behind Romney in Arizona polls, according to Real Clear Politics. The New York Times' Nate Silver gives Romney a 99 percent chance of winning in the state, and a 55 percent chance of winning in Michigan.
What happens: Wailing, gnashing of teeth in the Romney campaign and maybe among Republican officials. Arizona is like "an orphan in the desert," the Los Angeles Times' Mark Z. Barabak writes, and as Politico's James Hohmann notices, Arizona's biggest newspaper doesn't even have a front page story about the election Tuesday. So if Santorum were to have a very unlikely double victory, you could expect a lot of headlines with words like "shocker" and then some analysis about "momentum." Pundits would have plenty to talk about for the next few days about how the Romney campaign will respond, but they've been saying that for weeks now anyway. 
Chance the primary will really, really be overVery low.

Outcome: Santorum wins Arizona, Romney wins Michigan.
Likelihood: Extremely low.
What happens: Some pollsters lose their jobs. 
Chance the primary will really, really be overVery low.