Tuesday’s runoff pits two factions of the Republican Party against each other in one of the reddest states in the country, with Guest defending the Reagan-era GOP aimed at promoting fiscal restraint, and Cassidy—albeit in a somewhat half-hearted manner—championing an emerging family-oriented economic populism aimed at curbing what he calls the “destruction of the American family.” Guest said he worries that Reagan-era Republicans are a dying breed, and points to Cassidy’s prior commitments to sweeping social net policies as a sign that the GOP is moving into dangerous territory. “They are positions that no Republican who was running for Congress...