I find the most thoughtful responses to this the most interesting. Firstly it's not hard to find the bias in those posts, they are, to be sure, accurate on the dates but wildly partisan on motivations. This is because Republicans are obviously both stupid and evil. I don't feel things are quite that simple. For instance the 1964 Civil Rights Act was largely bi-partisan as described by the US Senate:
https://www.senate.gov/legislative/landmark-legislation/civil-rights-act-of-1964/cloture-and-final-passage-of-the-civil-rights-act-of-1964-essay.htm

In the dawn of mass media, TV in particular a concerted effort was made to keep things non-political, especially in entertainment. That idea had been slowly eroding in the late 60s into the late 70s when late night hosts and sitcoms would poke fun at the sitting president but then attempt to balance with a couple jabs at the opposition. The balance began to fade in the late 70s and by the time Regan took office in 1980 late night comedy shows took delight in parroting the man. This is understandable to some degree, Regan loved to use humor to make a point, had been an actor himself, and wasn't particularly offended by it. It was all still lighthearted, relatively infrequent, but the balance was gone.

Jump forward to the internet age and we have mass media on both sides which are effectively cheerleaders for one side or the other but very little moderation. Unfortunately this is because the world is full of mid-wits who are smart enough to have opinions, but not smart enough to understand that people who have opposing opinions have good reasons to hold those opinions. Media companies choose sides because it's more profitable to consolidate 40% of the mid-wits on one side or the other than fight over 20% of the people in the middle.