I personally think this is simply a result of how much harder the media is pushing that divide (for all of their various purposes). I actually spent some time last year researching this, because I thought I might have just become an old man thinking how great things used to be. I started reading old news stories fairly randomly, from the present time all the way back to the Vietnam era (and a few rabbit holes to earlier times). The first thing that surprised me was the amount of link rot that exists. I always knew intellectually that it was a problem, but wow. It's bad. The second thing that I found was that indeed, the media hammers on the "us-versus-them" political divide of American politics much, much harder nowadays than even ten years ago. I think Fox News was really the turning point. It opened the flood gates. I always remember thinking how "extreme" Fox News was, but I challenge anyone to look up a few of their older stories from the middle of the last decade. It's child's play compared to what pretty much every media outlet is doing today. You can hardly read a recent news story from just about anywhere without being told how it's supposed to fit into our political worldview, and how we should feel about it, and why it's good/bad/stupid/amazing/"terrifying". And so of course, because of this, people are just responding to the programming. Creating the world they're led to believe we they live in. I think it really is that straightforward.
Did you just look at print? Talk radio has been hammering this since the late eighties. Hell you can probably draw a line straight from the "Moral Majority" shit in the seventies, to where we find ourselves now. I suspect this has always been a big part of American culture, but it's being magnified now either by new tech or malicious actors or both.
Ah, I apologize, I didn't keep notes or save links or anything of the sort, and I keep kicking myself for it. I'm usually pretty good about taking notes just out of my regular habit of doing research, but it was so casual, and I didn't think it would end up taking as much of my time as it did. It's a pretty easy formula to replicate, though. I picked current events that I could remember -- intervention in Kosovo, Bill Clinton's sex scandal, Berlin Wall, first election of Putin, Enron scandal, those kinds of things -- and just started looking up stories, and asked my parents and older friends to help me with events I wasn't old enough to remember before the 80s. I made sure to hit a "good" cross-section of the media outlets of the day.
I'd love to see the Bush/Gore 2000 election play out on social media. I was only a kid but the news coverage seemed pretty mild compared to how I imagine it would be if that happened in 2016.
Everything is partisan political now. What books you read, what films and TV series you watch, where you live, the definition of "political" itself, and to some extent even what internet archiving service you use. (In reality, left-leaning folks have used archive.is to save and mock conservative sites for some time too, but even though this happens across the board it's still normal and expected to think of this as a partisan political activity because everything is now.)