Federal administrative agencies are one of the primary policymaking venues in the United States. One of the core features of U.S. administrative practice is the notice-and-comment process in which agencies solicit, collect, and respond to comments from the public before issuing new regulations. In this paper, we develop a model of commenting based on three motivations—litigation preservation; agency persuasion; and expression—and analyze public comments to determine how features of the political environment, and specifically the president in power, affect the pool of commenters. We focus on the 2017 presidential transition, when there was both a change in Presidents and the party in control of the White House. We find that there were greater differences in the pool of commenters between administrations than within administrations and that interest groups tended to participate more when they were more closely associated with the party in power. Our findings support the view that many commenters use the public comment process for persuasive purposes, and not only to preserve litigation opportunities or for purely expressive reasons.