Based on Ba'th Party archival records, interviews, and secondary sources, this article aims to reconstruct and contextualize the story of Muhammad Muhammad Sadiq al-Sadr, the Shi'i cleric who led a grassroots religious movement in the 1990s that still plays a major role in Iraq. The article argues that the Sadrist movement and its project of social Islamization were a result of Sadr's enlistment of grassroots support to challenge his rivals in the Shi'i religious field during a leadership vacuum amid the decline of the clerical establishment's influence.