Uh, I beg to differ about when it started to happen.
I'm old enough to remember politics pre-1980, and post-1980.
Before the elections of 1980, it seemed that political affiliation made no more difference to most people, than that one person had brown eyes and the other person had blue eyes. There were disagreements and arguments, but it wasn't as if politics was a good enough reason for people not to be friends.
For the record, I was in college about this time.
After November 1980, I suddenly noticed politics had started to "count;" that they were a determinant, a criteria, in establishing and maintaining associations and friendships.
Which bothered me at the time, but I figured it would pass.
It never did.
The Democrat loss in 1980, and of such epic proportions, seemed to have permanently embittered the losers, and 1980 was a very long time ago now, more than a generation.
The blame for excessively divisive partisanship lies squarely on the shoulders of the other side; I am not aware of a single case where a Republican or a conservative has been unwilling to extend the hand of friendship to a Democrat or liberal--because after all, they each may have 10,000 characteristics, only 1 of which is that they differ on politics, and the other 9,999 might be beneficial, of use, to both.
I am aware however of far too many cases where the hand of friendship has been bitten by the other side.
This phenomenon has been noted by others than just myself, and even in books.
The blame lies on the other side, and until they grow up, their bitterness is always going to discourage association and friendship on matters other than politics.