Despite the rapid growth of online political advertising, the vast majority of scholarship on political advertising relies exclusively on evidence from candidates’ television advertisements. The relatively low cost of creating and deploying online advertisements and the ability to target online advertisements more precisely may broaden the set of candidates who advertise and allow candidates to craft messages to more narrow audiences than on television. Drawing on data from the newly-released Facebook Ad Library API and television data from the Wesleyan Media Project, we find that a much broader set of candidates advertise on Facebook than television, particularly in down-ballot races. We then examine within-candidate variation in the strategic use and content of advertising on television relative to Facebook for all federal, gubernatorial, and state legislative candidates in the 2018 election. Among candidates who use both advertising media, Facebook advertising occurs earlier in the campaign, is less negative, less issue focused, and more partisan than television advertising. ∗Except where noted in the text, analyses presented were preregistered (https://osf.io/3px5b) prior to the release of the Facebook ad library. The Wesleyan Media Project acknowledges funding from the John S. and James L. Knight Foundation and Wesleyan University. We are grateful to Laura Baum, Dolly Haddad, Colleen Bogucki, Mason Jiang and the numerous undergraduates across our institutions for their efforts on this project. We thank Amanda Wintersieck, Devra Moehler, and seminar participants at APSA, the Princeton CSDP American Politics seminar, the University of Maryland, and the Wesleyan Media Project Post-Election Conference for comments on previous versions. †Associate Professor of Government, Wesleyan University ‡Professor of Government and Legal Studies, Bowdoin College §Assistant Professor of Political Economy, Stanford Graduate School of Business ¶Associate Professor of Political Science, Emory University ‖Thomas S. Foley Distinguished Professor of Government and Public Policy, Washington State University How does the medium of political communication affect the message, if at all? A glance at the landscape of US political media suggests some connection between the two, with right-wing outlets dominant on talk radio and cable news, and successful new digital-native outlets generally leaning left. In the comparative context, campaigns in democracies where broadcast media are more centralized and public-owned are more programmatic and partycentered than in those with more fragmented viewer markets (Plasser and Plasser 2002). Of course, these are pure correlations, and it is entirely possible that these associations between medium and content simply reflect the demographic profile of the audience, or common consequences of varying political cultures. Nonetheless, the dramatic technological changes experienced over the past 15 years have real potential to shift the strategic landscape of campaign communication, and thereby alter the content of campaign messaging that voters receive. In particular, the mass adoption of the Internet, smartphones, and social media have moved the technological frontier of mass communication in two strategically important ways. First, social media platforms substantially lower the cost of advertising, expanding the set of candidates for whom advertising and thus the potential to reach voters and seriously contest an election is a real possibility. Second, and perhaps even more consequential, social media platforms offer vastly more precise targeting capabilities than legacy broadcast media. This feature of social media could allow campaigns to strategically tailor messages to narrowly-defined audiences, a capability with the potential to undermine democratic accountability. Or perhaps some deeper psychological connection between preferences for medium and preferences for political ideology (Young 2019). The low cost to post ads on social media is not without some complicating factors. For example, some media coverage of the 2020 Democratic presidential primary noted that the competition among over 20 candidates for ad space on Facebook, in part driven by the need to meet unique donor thresholds to participate in early debates, meant that prices from Facebook were much higher than what many campaigns expected to pay. Those costs often meant that campaigns were spending more on social media than what those efforts were raising in online donations. Still, the price relative to TV remains much lower. See Egkolfopoulou (2019). For example, in the classic model of Ferejohn (1986), voters’ ability to use the threat of losing reelection to control incumbent behavior hinges on their observing a common performance signal; if the performance signals are individual-specific, voters’ power over incumbents evaporates. Wood and Ravel (2018) discuss the normative consequences of microtargeting with a particular emphasis on how democracy can be harmed when citizens are only exposed to political appeals from the candidates and campaigns that they are predisposed to support.