Over the last few decades, the American public has become more strongly partisan, affectively polarized, and sorted (i.e., aligned their partisanship with their policy preferences). This article presents a theory of partisanship in contemporary American politics as rooted in “the prejudiced personality”: need for closure. Based on uncertainty‐identity theory and the theory of groups as epistemic providers, I argue that today's polarized parties are more appealing to individuals high in the need for closure, causing them to (1) identify strongly with their party, (2) to be intolerant of members of the opposing party, and (3) to conform to partisan authorities (i.e., to sort). The current article examines both observational and experimental data in assessing the effects of need for closure on partisan strength, affective polarization, and sorting. I conclude by discussing the normative implications of a mass public driven not by ideology or worldviews but by a group‐centric and prejudiced cognitive style.