This article examines the sponsorship and content of mass comment campaigns in administrative rulemaking in the United States. Mass comment campaigns consist of identical and near‐duplicate comments sponsored by organizations and submitted by group members and supporters to government agencies in response to proposed rules. Drawing from research on interest group lobbying, it is posited that organizations of all types sponsor mass comment campaigns, but that campaigns submitted by regulated entities (i.e., industries) are more substantive than campaigns generated by beneficiaries of stringent regulations (e.g., environmental advocacy groups).