Popular dissatisfaction with representative democracy has encouraged governments and legislatures worldwide to experiment with democratic innovations. However, despite calls for a ‘systemic’ approach to the study of democratic engagement and participation, empirical knowledge is limited about the diffusion of democratic innovations within civil society, and, in particular, about the connective mechanisms that bring the ‘voice’ of citizens to the ‘ears’ of political elites. This article responds to this gap, presenting original empirical research examining the UK House of Commons’ e-petitions system. This research maps public engagement with parliamentary e-petitions across a range of expressive spaces, and highlights the facilitative role of non-institutional intermediaries. However, it also underlines the predominant role of institutional actors in structuring public participation, and shows that effective transmission between the informal public and formal political spheres remains contingent on both ‘designed-in powers’ of institutional coupling and ‘developed practices’ of public engagement. Through this analysis, the article makes an important contribution to debates concerning democratic innovations, political participation, and institutional design.Supplementary InformationThe online version contains supplementary material available at 10.1057/s41293-022-00208-9.