1. "breakdown in recent years."
It is really a historical more than psychological question. Now, the truth is, there have been lots of horrible times in history, even American history. I mean, we fought the bloodiest war in American history (Civil War) because North refused to accept Southern slavery and secession. Insulting each other does not really compare.

Even the late 1960s/early 1970s were, in some ways nastier. They were certainly more violent, with riots all over the place, National Guard killing protesting students, and the like.

So that all argue against my points. Still, I was thinking more like, "in the last 40 years or so." I find the Republican Convention chant, "lock her up," chilling.

And, speaking experientially, more than from data, I have never felt campus free speech and academic freedom so under threat at any time in my career, which is over 30 years.

It feels like an only somewhat milder version of the rightwing anti-communist fever of the 1950s, where people were blacklisted and fired. There are not quite such extreme witch-hunts yet, but ... people's careers have been obstructed and damaged by the modern, mostly though not exclusively, leftist weaponization of speech (read, e.g., Dreger's book, Galileo's Middle Finger or Pinker's The Blank Slate for lots of examples).

2. References on the hostility, simplifications and exaggerations produced by Us vs. Them thinking

http://www.rci.rutgers.edu/~jussim/CDPS.pdf
(on stereotype accuracy, but see the section on political stereotypes).

https://ed.stanford.edu/sites/default/files/party_over_policy_0.pdf
(many people blindly follow the party they identify with)

http://crawford.pages.tcnj.edu/files/2011/12/Brandt-Reyna-Chamber-Crawford-Wetherell-CDPS.pdf
Libs and Cons pretty much despise each other

https://www.researchgate.net/profile/John_Chambers3/publication/270396011_Perceiving_Political_Polarization_in_the_United_States_Party_Identity_Strength_and_Attitude_Extremity_Exacerbate_the_Perceived_Partisan_Divide/links/54a9ceba0cf257a6360d59ef.pdf
-- this is the manuscript version, but I am pretty sure it has been published, so, if it is important, you should track it down. Partisans exaggerate each others' views, ala my first paragraph under Background and Context