Presidential debates allow candidates to send a message directly to voters. We use an experimental design complemented with a content analysis of all presidential debates in 1992, 2004, and 2008 to explore how candidates should and do use agenda setting, framing, and message tone to shape the agenda in debates. We find that candidates are differentially attentive to various topics, depending on the comparative advantage provided by the topic. Yet, this agenda control occurs only at the margins because topic salience in public opinion predicts candidate attention and conditions voters’ receptiveness to debate rhetoric. Our findings thus suggest that topic salience constrains candidates’ abilities to focus the agenda strategically.