In authoritarian systems, ethnic power-sharing arrangements include important ethnic groups in government and decision-making while putting restraints on political competition. However, under conditions of democratization, we might expect power-sharing arrangements to fragment as political parties seize opportunities to expand their base and appeal across ethnic lines. This article draws from the case of Malaysia, where multiethnic coalitions built around ethnic parties ruled for 61 years but where increasing electoral competitiveness has destabilized coalition politics. I focus on the Democratic Action Party (DAP), one of the country's most successful parties, which has sought to build a more multiethnic support base. I show that its attempts have been stymied by enduring norms of ethnically informed coalition building and efforts to protect existing ethnic bases by both rivals and allies. The findings shed light on the barriers to ethnic party adaptation and on why power-sharing practices remain so enduring, even in more fluid and democratic political environments.