ANN ARBOR, Michigan — As he watched last week's Republican debate from his apartment in University of Michigan's married student housing, Derek Osborne could hardly contain his disgust every time Rick Santorum came on TV. "He represents a part of the Republican party that drives me absolutely crazy," he seethed. Osborne, a Ph.D candidate, is a Mitt Romney supporter. He's also a moderate Republican who is alarmed at what he sees as the anti-intellectual fervor—and undeniable political power—of the Tea Party movement. As the latest candidate to emerge as the main populist rival to Romney, Santorum has become the primary...