Not to excuse the current crop but, the nastiness has actually been around a very long time. I have seen, for example, newspaper articles from the 19th century that were absolutely full of personal insults and attacks on the morals of the political oppostion. I think it is more subtle today which might make it more effective. When I hear or read outlandish claims it is usually pretty easy for me to dismiss them. When those claims are couched in terms that sounds reasonable it makes them harder to discredit.

Most of the negative material now comes from paid advertisements. Back in the day it came in the form of news since papers tended to be mouthpieces for a particular party. It was known which papers were shilling for which parties and I have wondered how much cross party readership there was, in this part of the U.S. an area often had two or three papers from which to choose.