Why do independent judiciaries fail to constrain democratic erosion and to institutionalize judicial constraints on the executive over time? Conventional wisdom holds that independent courts can defend democracy, especially when fragmentation of power among political parties protects judicial independence. I argue instead that courts can enable democratic erosion, even when power is fragmented. I theorize that the partisan biases of the judiciary shape both whether courts constrain aspiring autocrats in the short term and whether judicial constraints on the executive become stably institutionalized over the long run. When courts have partisan biases against incumbent governments, they are more likely to engage in aggressive judicial activism—thereby encouraging a “judicialization” of the opposition’s strategy and encouraging government attacks on the courts. Connecting short-term strategic interactions with long-run patterns, I argue that judicial constraints on the executive fail to institutionalize when historical legacies of repression tie the judiciary to one side of a polarized partisan divide. I test this theory in Turkey during the 2000s, where an independent judiciary aggressively ruled against the elected government yet failed to limit democratic backsliding. The paper leverages an original quantitative dataset of 3,000 decisions issued by Turkey’s Constitutional Court as well as interviews with high-ranking Turkish judges.