The rise of conservative media and their assault on anything objective in news media presentation dates back to the 1940s. See Human Events magazine which began in 1944 (I think), Henry Regnery and William F. Buckley, Jr in the 1950s, Barry Goldwater's 1964 campaign openly attacking journalist's objectivity as a mask for their liberal bias, Edith Efron's "scientific" book The News Twisters which became a NYT bestseller in the late 1960s because Nixon and friends bought shedloads of copies and in the 1970s the rise of Point/Counterpoint on 60 Minutes as well as radio programs that began to be specifically devoted to conservative political points of view.The removal of the FCC Fairness Doctrine in 87-88....somewhere around there.
Then we got Ronald Reagan and Pandora's Box was fully opened.