Scholars have long argued that political participation is determined by institutional context. Within the voter turnout literature, the impact of various institutional structures has been demonstrated in numerous studies. Curiously, a similar context-driven research agenda exploring the correlates of nonelectoral participation (NEP) has not received the same attention. This study addresses this lacuna by testing a political opportunity structure (POS) model of citizen activism across 24 old and new democracies using International Social Survey Programme 2004: Citizenship (ISSP 2004) data. Using a multilevel modeling approach, this study tests a competition versus consensus conception of how decentralized institutions determine NEP. This research demonstrates that states with more competitive veto points operating through systems of horizontal and territorial decentralization increase individual NEP. In addition, it interacts with social mobilization networks to promote greater citizen activism: Institutional context counts only when citizens are mobilized.