This article concerns the organizational expansion undertaken by the opposition party, Chadema, in Tanzania between 2003 and 2015. It argues that Chadema's extensive party-building enabled it to mobilize on the ground. These organizational developments, as much as elite action, underpinned recent changes in the party system and the opposition's improved showing in recent elections. Chadema established branches even though many of the prerequisite circumstances typically recognized in the literature were absent. This makes Chadema a deviant case and this deviance has implications for the historical institutionalist literature on party-building. This article complicates Rachel Riedl's account of state substitution. She links the incorporation or substitution of social actors to different paths of party system institutionalization. This article demonstrates that the character and consequences of state substitution depend upon the balance of power between state and social actors. It also builds on accounts by Adrienne LeBas and others that when social actors are strong, they can endow opposition parties with resources which make branch establishment possible, and when they are weak, they can only act as surrogate party branches. This article illustrates that when social actors are absent from partisan politics, parties have no way to organize except by founding green-site branches.