Although conspiracy theories have long been a staple of American political culture, no research has systematically examined the nature of their support in the mass public. Using four nationally representative surveys, sampled between 2006 and 2011, we find that half of the American public consistently endorses at least one conspiracy theory and that many popular conspiracy theories are differentiated along ideological and anomic dimensions. In contrast with many theoretical speculations, we do not find conspiracism to be a product of greater authoritarianism, ignorance, or political conservatism. Rather, the likelihood of supporting conspiracy theories is strongly predicted by a willingness to believe in other unseen, intentional forces and an attraction to Manichean narratives. These findings both demonstrate the widespread allure of conspiracy theories as political explanations and offer new perspectives on the forces that shape mass opinion and American political culture.