In this dissertation I investigate how mass media outlets might act politically by using their agenda-setting power, i.e. by affecting through their coverage the importance readers and viewers attach to different issues. According to the issue ownership hypothesis, as introduced by Petrocik [1996], the choice of the topic being covered can have relevant persuasive effects, to the extent that citizens think that a given political party is better at handling problems related to it, compared to its opponent(s). I first construct a simple model of electoral competition with two candidates, two issues and one newspaper, which has room for only one story to be published. The model shows how rational citizens are influenced in their voting choice by the story featured on the newspaper, but tend to overreact to stories that go contrary its known editorial policy. From an empirical point of view, I first study the coverage devoted by the New York Times to Democratic and Republican issues, during the 1946-1997 period. I find that the Times has a Democratic partisanship with some watchdog aspects, since during presidential campaigns it systematically gives more emphasis to the Democratic issues of civil rights, health care, labour and social welfare, but only so when the incumbent president is a Republican, i.e. he is perceived as weak on those issues. Building on a joint work with James Snyder and Valentino Larcinese, I then study the coverage of economic issues by a large sample of U.S. newspapers during the last decade. I find that newspapers with pro-Democratic endorsement pattern systematically give more coverage to high unemployment when the incumbent president is a Republican than when the president is Democratic, compared to newspapers with pro-Republican endorsement pattern. This result does not seem to be driven by the partisanship of readers.
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