  Social media are decentralized, interactive, and transformative, empowering
users to produce and spread information to influence others. This has changed
the dynamics of political communication that were previously dominated by
traditional corporate news media. Having hundreds of millions of tweets
collected over the 2016 and 2020 U.S. presidential elections gave us a unique
opportunity to measure the change in polarization and the diffusion of
political information. We analyze the diffusion of political information among
Twitter users and investigate the change of polarization between these
elections and how this change affected the composition and polarization of
influencers and their retweeters. We identify "influencers" by their ability to
spread information and classify them into those affiliated with a media
organization, a political organization, or unaffiliated. Most of the top
influencers were affiliated with media organizations during both elections. We
found a clear increase from 2016 to 2020 in polarization among influencers and
among those whom they influence. Moreover, 75% of the top influencers in 2020
were not present in 2016, demonstrating that such status is difficult to
retain. Between 2016 and 2020, 10% of influencers affiliated with media were
replaced by center- or right-orientated influencers affiliated with political
organizations and unaffiliated influencers.
