“I must be about my father’s business,” Tagg Romney announces. It is a warm summer evening last July. He stands before a packed classroom of some 50 adults at his family’s long-time spiritual home, a meetinghouse of the Church of Jesus Christ of Latter-day Saints (LDS), located in the affluent Boston suburb of Belmont. Looking every bit the part of his father’s son, with the same graying sideburns and chiseled features, Tagg is teaching adult Sunday school. And while he is in fact quoting Jesus in the garden of Gethsemane, he is also—as the oldest Romney son and key surrogate for Mitt’s presidential campaign—certainly engaged in his father’s business these days.

But today is Sunday. And Sunday is a day that the...