Legislative capture, wherein an autocratic party wins control of a country’s legislature, is a critical stage of democratic backsliding. This thesis uses a mixed-methods approach to investigate how different types of party systems contribute to legislative capture by an autocratic party, and thereby enable democratic erosion. A quantitative analysis of 17 cases of democratic backsliding in post-communist Eastern Europe sets the stage for an analysis of four cases: Hungary, Poland, Czechia, and Slovakia. In Hungary and Poland, bipolarized party systems—defined by party duopolies and unidimensional polarization—allowed autocrats to win control of each country’s legislature and to use legislative and executive power to weaken democratic institutions. In Czechia and Slovakia, by contrast, multipolarized party systems—defined by high fragmentation and multidimensional polarization—prevented legislative capture by autocratic parties, particularly by forcing would-be autocrats to form precarious governing coalitions. The central argument of this thesis is thus that party systems in post-communist Eastern Europe played a powerful and hitherto underappreciated role in encouraging or discouraging legislative capture by autocratic parties, which in turn contributed heavily to the acceleration or thwarting of democratic backsliding. This thesis concludes by demonstrating the critical role of bipolarized vs. multipolarized party systems in facilitating or discouraging legislative capture, and thus democratic backsliding, in a range of other cases around the world.