There has been an intense scholarly debate about what caused the unprecedented Islamist mass demonstrations in Indonesia in late 2016. Some scholars have argued that increasing intolerance and conservatism among the Muslim population are responsible, while others have disputed such notions, claiming that there is no evidence of widespread support for an Islamist agenda. In this article, we analyse a unique set of polling data to show that a) conservative attitudes among Indonesian Muslims were declining rather than increasing prior to the mobilisation, but that b) around a quarter of Indonesian Muslims do support an Islamist socio-political agenda. Importantly, we demonstrate that this core constituency of conservative Muslims has grown more educated, more affluent and better connected in the last decade or so, increasing its organisational capacity. We argue that this capacity was mobilised at a time when conservative Muslims felt excluded from the current polity, following the end of a decade of accommodation.