  The emergence of online political advertising has come with little
regulation, allowing political advertisers on social media to avoid
accountability. We analyze how transparency and accountability deficits caused
by dark money and disappearing groups relate to the sentiment of political ads
on Facebook. We obtained 430,044 ads with FEC-registered advertisers from
Facebook's ad library that ran between August-November 2018. We compare ads run
by candidates, parties, and outside groups, which we classify by (1) their
donor transparency (dark money or disclosed) and (2) the group's permanence
(only FEC-registered in 2018 or persistent across cycles). The most negative
advertising came from dark money and disappearing outside groups, which were
mostly corporations or 501(c) organizations. However, only dark money was
associated with a significant decrease in ad sentiment. These results suggest
that accountability for political speech matters for advertising tone,
especially in the context of affective polarization on social media.
