April 3rd is the Wisconsin primary (along with MD, and DC), and if Santorum can’t take that, it’s hard to see how he goes on. Well, goes on without making a fool of himself, that is (see also “Newt”).
Romney is heavily favored to take MD. Also, DC, where Santorum isn’t on the ballot (never turned in the required signatures, apparently assuming that DC isn’t really a very important place, politically). So Wisconsin is critical, if Santorum wants to have even a Hail Mary chance of winning.
After April 3rd, the next round of primaries isn’t until April 24th, 3 weeks later. And while that date includes Santorum’s home state of Pennsylvania, it’s gonna be tough to hang in and wait for three long weeks, having gone “0 for April.”