(OPEN ACCESS) This paper establishes moral duties for intermediaries of political advertising in election campaigns. First, I argue for a collective duty to maintain the democratic quality of elections which entails a duty to contain some forms of political polarization. Second, I show that the focus of campaign ethics on candidates, parties and voters—ignoring the mediators of campaigns—yields mistaken conclusions about how the burdens of the latter collective duty should be distributed. Third, I show why it is fair to require (...) intermediaries to contribute to fulfilling this duty: they have an ultimate filtering position in the campaign communication process and typically benefit from political advertising and polarization. Finally, I argue that a transparency-based ethics of campaign advertising cannot properly accommodate a concern with objectionable polarization. By contrast, I outline the polarization-containing implications of my account, including a prohibition on online targeted advertising, and intermediaries’ duties to block hateful political advertising. (shrink)