  Political campaigns are increasingly turning to digital advertising to reach
voters. These platforms empower advertisers to target messages to platform
users with great precision, including through inferences about those users'
political affiliations. However, prior work has shown that platforms' ad
delivery algorithms can selectively deliver ads within these target audiences
in ways that can lead to demographic skews along race and gender lines, often
without an advertiser's knowledge.
  In this study, we investigate the impact of Facebook's ad delivery algorithms
on political ads. We run a series of political ads on Facebook and measure how
Facebook delivers those ads to different groups, depending on an ad's content
(e.g., the political viewpoint featured) and targeting criteria. We find that
Facebook's ad delivery algorithms effectively differentiate the price of
reaching a user based on their inferred political alignment with the advertised
content, inhibiting political campaigns' ability to reach voters with diverse
political views. This effect is most acute when advertisers use small budgets,
as Facebook's delivery algorithm tends to preferentially deliver to the users
who are, according to Facebook's estimation, most relevant.
  Our findings point to advertising platforms' potential role in political
polarization and creating informational filter bubbles. Furthermore, some large
ad platforms have recently changed their policies to restrict the targeting
tools they offer to political campaigns; our findings show that such reforms
will be insufficient if the goal is to ensure that political ads are shown to
users of diverse political views. Our findings add urgency to calls for more
meaningful public transparency into the political advertising ecosystem.
