It’s pretty simple.
Mitt Romney needs 52% of the remaining delegates, only a maximum of 122 of which are available from winner-take-all states. If he doesn’t start winning delegate majorities now, he will not be able to win a first-ballot majority, and will probably have to put one of his current rivals on the ticket and make major platform concessions as well.
PPP is suggesting Santorum may be leading in Michigan, but Michigan has less delegates actually in play than Arizona and because it is proportional, whereas Arizona is winner take all (on the same day). If any kind of polling shows Santorum has a chance there, he should get there and try and claim the 29 delegates there, instead of seeing if he can get 15 instead of 10 in Michigan. It would also show that he is more than a regional candidate.
If Santorum can maintain momentum on that day into Super Tuesday, where Romney is running into trouble in Georgia, Oklahoma, and Ohio, Santorum could claim the actual delegate lead.