  Social media play an increasingly important role in political communication.
Various studies investigated how individuals adopt social media for political
discussion, to share their views about politics and policy, or to mobilize and
protest against social issues. Yet, little attention has been devoted to the
main actors of political discussions: the politicians. In this paper, we
explore the topics of discussion of U.S. President Obama and the 50 U.S. State
Governors using Twitter data and agenda-setting theory as a tool to describe
the patterns of daily political discussion, uncovering the main topics of
attention and interest of these actors. We examine over one hundred thousand
tweets produced by these politicians and identify seven macro-topics of
conversation, finding that Twitter represents a particularly appealing vehicle
of conversation for American opposition politicians. We highlight the main
motifs of political conversation of the two parties, discovering that
Republican and Democrat Governors are more or less similarly active on Twitter
but exhibit different styles of communication. Finally, by reconstructing the
networks of occurrences of Governors' hashtags and keywords related to
political issues, we observe that Republicans form a tight core, with a
stronger shared agenda on many issues of discussion.
