The rise of wedge social issues is mostly to blame imo, specifically abortion. There are so many people that think the other party is a bunch of terrible people that are going to hell. Of course politics will be divisive when that's how people think.
Abortion policy wouldn't be nearly as toxic of an issue if it hadn't been unilaterally mandated by the Supreme Court in 1973 but was allowed to evolve by natural democratic processes.
Honestly? A growing dread and sinking conclusion that the country's best days are behind it and that the country is going to the dogs, which is shared by people on both sides of the aisle and has been for decades (at the very least since 9/11).
Have you talked to many liberal Boomers lately? A lot of them seem to think that "the decline" started on November 22, 1963 and has only gotten worse since.
It's funny you say that, although not for the obvious reasons. In a lot of ways, 1963 was the high water mark for American community based on a whole host of measures; ever since then, participation in all manner of American civic and communal life has eroded precipitously. We volunteer less, we vote less, we join organizations less, we generally do fewer things in person together, we interact face-to-face less frequently than ever. Much of this decline is associated with the rise of television in the home and suburbanization.
It's not hard to see trends like this and think we've lost something fundamental in the way we relate to our fellow man.