Innovative ideas often need champions to advocate for them. The study of policy innovation, however, has often focused on enactments rather than earlier stages of the legislative process, in which the actions of a bill’s advocates may be especially crucial. In this paper, I ask who the champions of policy innovations are, focusing on two categories of legislators whose expertise makes them likely advocates: individuals with relevant professional backgrounds and individuals serving on relevant legislative committees. However, I also argue that the impact of expertise is conditional: as policies diffuse more widely, the role of expertise fades in importance, and champions can come from the broader legislative membership. To test these claims, I examine bill authorship and cosponsorship across 14 criminal justice policies between 1993 and 2004. I find that committee membership and professional background are indeed powerful predictors, but that their impact decreases as more states adopt the policy.