The role of the 30% threshold for Islamic parties: A fast-growing middle class and religion-based political preferences in Indonesia

Recently, Indonesia's middle-class Muslims have come under suspicion for strengthening the religious foundation of their political preferences. However, while this suspicion has been triggered by their increasingly confident expressions of Islamic identity in political, social and economic spheres, these public expressions do not consistently translate into electoral outcomes. Despite the significant growth of the middle class, the proportion of votes cast for Islamic parties, encompassing both pro-shari'a or Islamist and Islam-inclusive parties, has declined from 37% in 1999 to 30% in 2019. Utilising panel data from 397 districts and cities spanning three recent general elections (2009, 2014 and 2019), this study examines the association between the middle class's proportion of the total population and the vote share of Islamic parties. Our fixed-effect estimations validate an inverted U-curve relationship between the middle class's share and the vote share of pro-shari'a or Islamist parties, with the turning point occurring at approximately 29% of the middle class's total population. In regions with a lower percentage of the middle class, the vote share of Islamist parties rises in parallel with middle-class growth. Conversely, at a higher level, a growing middle class curtails the vote share of Islamist parties. This suggests that Indonesia's contemporary social structure is undergoing desecularisation in various ways, contingent on levels of modernisation. Notably, this desecularisation process does not imply a movement towards establishing an Islamic state. Indonesia's Muslims are moderate and more inclined to support secular, national or Pancasila-based parties, which tend to be inclusive and secular in their approach.