It's fairly obvious that a Romney loss would end his campaign while a win would keep him afloat. McCain's stakes are just as serious. Take a look at the RCP polling averages for South Carolina, Nevada, Florida, and Feb. 5th. He is at worst tied in each of them and leads in the most recent (i.e. post-New Hampshire) polls. A win in Michigan could put him solidly ahead in Nevada AND South Carolina with just four days for Giuliani (in Nevada) and Huckabee (in South Carolina) to recover. Two wins on Jan. 19th and he'll roll into Florida the winner of 4 of the first 5 contests, including 4 in a row. That kind of momentum would likely take his minuscule lead in Florida and put it in the deep freeze. Feb. 5th would be a mere formality, a coronation of McCain as the nominee. The scenario is sort of like Iowa was for the Democrats. If the national frontrunner had won, she would have moved very close to locking up the nomination after only one state. Instead, the underdog won and now the decisive event has been put off until at least Feb. 5th.
Oh, and don't let this post lead you to think that I give a damn for the whole state of Michigan, because I don't.