You know, the "us vs. them" mentality started after 9/11 when Jr. said that if you're not with us, you're with the terrorists. That is what started the division because people started to look for others who didn't think like them so they could keep a watch over them, just in case.
Then? The division was further exacerbated when Obama won the presidency and the Republicans (led by Mcconell, Boehner) then stated THE FIRST DAY that their whole goal was to make Obama a one term president.
After that? They used the "us vs. them" thinking to divide the parties to the point where we have a huge partisan divide between Dems and Republicans.
Gimmie back the 80's and the 90's, when even though both sides didn't like each other, they at least worked together for the betterment of this country.