Agenda Setting by News and by the Audience in a News Portal Panel Experiment

News can powerfully influence audience issue priorities, but the classic experiments on agenda setting occurred decades ago, prior to major changes in media. This calls not only for updated replication, but also for implementing advancements already made in recent non-experimental agenda-setting scholarship. Particularly, the new wave of agenda-setting experimentation should question the original assumptions that news is unique in its agenda-setting power, that issues are independent, and that effects are uniform across party lines. A 12-day experiment embedded in a purpose-built online news portal tested effects of the news agenda and the "user agenda." Participants were randomly assigned to encounter more or fewer real, timely stories about three topics (the news agenda) and altered rankings of stories about two other topics in a trending or recommended sidebar (the user agenda). Both agendas had significant effects regardless of partisanship. Contrary to the issue independence assumption but consistent with group threat theory, a user agenda emphasizing anti-Black racism increased immigration importance, particularly among Republicans.