In the interest of enlarging their constituencies, social movements often broaden mobilization efforts beyond the directly aggrieved, beneficiary populations. The authors examine this process through an analysis of a movement against unconventional oil and gas development (UOGD or “fracking”) in Illinois. Using data on more than 37,000 public comments submitted during a regulatory review of fracking, the authors examine the composition of the antifracking movement’s constituency. Individuals residing outside the targeted development region submitted the vast majority of the comments opposing UOGD. Analyses reveal that these participants tend to be Democratic partisans who were affiliated with one or more large environmental and progressive organizations. The study offers new insight into how environmental and progressive issue campaigns are organized in the contemporary U.S. context. The authors highlight how partisan discursive opportunities combine with recent organizational innovations in the social movement sector to enable nascent single-issue campaigns to tap into the mobilization potential of motivated partisans.