When one looks at the point in time when journalism became unapologetically leftist, it would be around the time of the Vietnam War.
All of the young campus communists graduated from college, and took their leftist beliefs with them into middle age.
They found themselves in the position where they could take over most institutions in order to implement their cancerous agenda (they became the teachers, the journalists, etc., and found themselves in a position to inculcate the youth of America with their malignant ideas.)
Once they stole our children away from us, it was only a matter of perpetuating this cycle. In my opinion.
. . . but if so, how did it happen so quickly? IMHO it was an accomplished fact long before then.
In the Vietnam era, journalists loved to quote students who prattled about "the Establishment" which controlled America instead of the people. But back in 1953 when Eisenhower was forming his cabinet there was a revealing (in historical retrospect) incident.
General Motors executive Charles Wilson was nominated to be SecDef and, in those more innocent times, he was within his legal rights to expect to be confirmed and to serve without first disposing of huge holding of GM stock. But at his confirmation hearing he was asked if that were not a conflict of interest between his interests as a GM shareholder and the nation's interest. He replied, "What is good for the country is good for General Motors."
Journalism modified the quote to read, "What is good for General Motors is good for the country," and make a huge ruckus over it. Wilson ultimately was confirmed, as I recall, but the incident was quite a big deal. And I suppose that our present draconian conflict-of-interest rules may ultimately trace back to it.
In historical retrospect it can be seen that tho the two formulations do not mean exactly the same thing, they do have the same subtext. The implication of the actual statement, and the naked meaning of the journalist-distorted one, is that General Motors was part of an illicit American Establishment. By modifying the statement and making a huge bruhaha over what Wilson had actually said, journalism revealed its true nature. What we saw then, all unknowingly, was the actual Establishment slapping down a pretender to the status of member of the Establishment.
Which is after all exactly what journalism did during Vietnam and, IMHO, ever more nakedly in defense of the Clinton Administration. And that Establishment is furious over the defeat (over its own strongest efforts) of Gore and determined to tarnish, nay damage, Mr Bush.
My thesis is that the affectation of objectivity by journalism is, in and of itself, corruption leading directly to the tendentious mess that we all understand for the bitter joke it is.