Being a student of history, I'm not aware of any studies regarding political discourse per se. I am aware of things that have been said by politicians about each other, or about one political party or interest group, but the current concern over the incivility of political discourse, while legitimate is without validation of progression. Said in more simple language, I do not think political discourse has gotten worse over the past decade, century or millennium. I'd guess it's about the same.
There have been some pretty bad things that been said in public in the past about political figures. What concerns people about un-civil political discourse (discourse is defined as oral expression or conversation)is two-fold. First is it irritates our sensibilities (incivility is a big "turn-off" for me)and second, what it may lead to, such as violence. It seems obvious that the the recent violence that occurred in Tucson was not in any way politically motivated. Even so, people cannot help to blame such irrationality on a rational thing and in this case, un-civil political discourse is getting the blame in some quarters.
It's the opinion of this writer that discourse of any kind can lead to action, good or bad. Certainly, history shows that rhetoric often leads to bad things. Look at Adolph Hitler, a mesmerizing speaker, but also look at Churchill. In both of these cases, however, it doesn't appear that either individual was particularly rude, and certainly not when they were giving a formal speech. The point to be made here is that discourse and/or rhetoric can lead to both good and bad action. I'm sure there are many cases where less well-known personages have given speeches or held conversations that may have led to serious destruction as well as inspired edification. In researching the historical elements of my book,The Fall and Rise, I learned that many leaders were assassinated; the most famous being that of Julius Caesar that took place in the Roman Senate.
What has definitely changed has been that what politicians say has been magnified many times over by the media. The media has done a lot of good, but in the context of political discourse, it does a disservice.