By the time Mitt Romney arrives in Wisconsin Friday – his first campaign stop in the state before its GOP primary on Tuesday – his challenger Rick Santorum will have already become a familiar face. Mr. Santorum has been in Wisconsin since last weekend and has dined, bowled, and played shuffleboard with residents in every pocket of the state. He’s even tossed a football at Lambeau Field, home of the Green Bay Packers. Santorum’s once-generous lead in Wisconsin is eroding. In February, the former US senator polled at 34 percent among the state’s likely Republican voters, while Mr. Romney, the...