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). These expectations are confirmed in the context of more than one thousand mass comment campaigns that occurred during Environmental Protection Agency rulemakings between 2012 and 2016. The analysis primarily focuses on data collected about the identity of organizations that sponsor mass comment campaigns and the content (i.e., length and complexity) of campaigns, and is supplemented by interviews conducted with organizations that sponsor campaigns. The findings suggest that mass comment campaigns are not a phenomenon meriting unique explanation, but rather occur in a manner similar to lobbying in other policymaking venues, such as lawmaking in Congress.