CHICAGO — On the issue Republicans tell pollsters is the most important of 2012, Rick Santorum these days has very little to say. His typical stump speech, an almost hour-long mixture of Orwellian fears of government overreach, history lessons about the French and Middle Eastern revolutions and suggestions that President Barack Obama would rather side with Islamists than Americans, has evolved into a tour de force of building fear in his listeners. Since winning primaries in Alabama and Mississippi, Santorum’s standard remarks have generally excluded an economic plan — the heart and soul of the message his chief GOP rival,...