Mark Levin's most recent book, Unfreedom of the Press, that I've recommended a number of times now, covers the history of how the press gave up any pretense to objective coverage of the news and became the partisan mouthpiece of the Left.
Which I consider to be right wing propaganda bias. From the coverage of Bernie Sanders in the press in 2016 and again now, I see very little coverage of his rallies and positions, some criticism that is misinformed and other slights. This is hardly what I would expect from a leftist media.
The main stream media is owned by corporations that are interested in corporation profits. Bottom Line. This is why - in my opinion - main stream media "news" has become infotainment, self-promotion of stations and fluff pieces on media people (movie stars etc) rather than real news. The more people watch the station the happier the corporate owners are. And if there is propaganda it is to promote corporate interests.
In politics today it seems that Eisenhower would be a "leftist socialist" politician because the republican party has move so far to the right and away from their traditional values. You see this again and again in comments from people that no longer register as republicans that the party has left them.
Enjoy