There was another key development in the early 1990s, with the development first of the 24-hour news cycle, then the partisan divisiveness of it. At the same time, political consultants began to focus on divisive language and coded name-calling, continuing a notion that dated back to the late 1960s -- a notable divisive time in American history.

So I can't tell whether this is just the natural growth of partisanship as reflected in new media, or whether there is an actual inflection point. There is a feedback causing an exponential growth, and it can be hard to tell the difference between an apparent sharp, sudden increase in an exponential curve, and a real external effect.