Whether a political topic is seen in a moral and moral-emotional light has significant implications for political decision-making and public opinion. However, where prior research has investigated who among different actors has the ability to influence which topics enter the agenda, we do not know how a political topic – once it has entered the agenda – becomes imbued with moral-emotional qualities. To study who drives the moralization of a topic, we zoom in on immigration discourse in Germany. We base our analysis on fine-grained social media data from politicians, political parties, newspapers, and members of the public. After employing a transformer model to identify moral and moral-emotional appeals, we use vector autoregression models to demonstrate the important role of political parties, and especially that of radical-right challengers, in setting the rhetorical tone of public discourse. The results inform theories of moralization and political entrepreneurship.