From 2000 to 2010, Bolivia and Peru underwent similar processes of political decentralization toward the meso level of the government. Three elections later in Peru and two in Bolivia, the ability of national political parties to articulate interests differs markedly between the two countries. Peru tends toward fragmentation with national parties incapable of participating or successfully competing in subnational elections, while in Bolivia, the Movimiento al Socialismo (MAS) – and other parties to a lesser extent – are increasingly capable of participating and winning subnational offices. This paper argues that, despite having undergone very similar institutional reforms, the difference between the cases can largely be explained by two “society-side” variables: the caliber of the political ideas in debate and political social density. The substantive quality of ideas in debate and a greater political social density have been crucial to the Bolivian trend, while their absence has lessened the possibility of anything similar occurring in Peru. In general terms, the article sheds light on the social conditions that favor party-building in a context of decentralization reform.