Between the 1960s and the 1980s, an “economic style of reasoning”—grounded in the discipline of economics but traveling well beyond it—became influential in Washington, and was institutionalized through legal and organizational changes. This new way of thinking had consequences for what policy options were considered and how policy decisions were made, and was particularly constraining for the left wing of the Democratic Party. Drawing from a new book that looks at how such changes played out across the domains of social policy, market governance, and social regulation, this talk will focus on what this transformation looked like in the realm of antitrust policy. Here, a domain that was once conceived of as balancing competing purposes, including promoting competition, limiting corporate power, and protecting small business, was rethought as focused on a single goal: protecting consumer welfare, understood as allocative efficiency. As this new approach was built into legal frameworks and decision-making processes in federal agencies, it narrowed the scope of legitimate debate in ways that persist to the present—with implications for our ability to address new forms of corporate power.