As Rick Santorum desperately tries to make a dent in Mitt Romney’s formidable delegate lead, he faces an unlikely obstacle on the primary calendar: his home state of Pennsylvania. Yes, Santorum is currently favored — though hardly a lock — to win the popular vote in the state he represented in Congress for 16 years. But Pennsylvania’s non-binding primary rules for distributing delegates raise the prospect that Santorum, who has said he’ll win the vast majority of the state’s delegates, could actually come away from next month’s primary empty-handed at a time when he can ill-afford it. Which means the...