Can either Mitt Romney or Rick Santorum meaningfully advance beyond the ideological and demographic beachheads of support they have secured in the marathon slog for the Republican presidential nomination? That’s probably the most important remaining question in the grueling race. And it will be voters like Mindy Meyer -- a surgical nurse from nearby Vandalia who drove with her husband to see Santorum in this small southern Illinois town on Saturday afternoon -- who answer it. Meyer liked everything she heard from Santorum on reducing federal spending and increasing pressure on Iran not to develop nuclear weapons. She even agreed,...