Exactly correct. It is precisely from the time of the Sullivan decision that the United States became a Mediacracy, where the Media, dominated by the Progressive ideology, became far more powerful than the political parties.
The Progressive ideology favored the Democrats, but most Republicans were Progressives as well.
The ideological domination of Progressives in the Media can be traced back to the FCC and the dominance in the Media from the radio networks which became the television networks.
It still blows my mind that our Congress members can lie, tell any lie in Congress and not be held accountable.