Before the 1950s, what you thought about health care, guns, or abortion had little to do with where your vote went. Then the modern two-party system came along—or was engineered. On a recent episode of The Gist, Mike Pesca spoke with Sam Rosenfeld, author of The Polarizers: Postwar Architects of Our Partisan Era, about the evolution of political parties in the U.S. Their conversation, which has been edited and condensed for clarity, is reprinted below.