1968. Race riots, white flight, hippies, anti-war protests, assasinations, hysteria about the world coming to an end, etc.
Imagine 1968 or even 2004 with social media .
I think 2016 just barely was more polarizing due to the mass media, if 1968 had all the social media we have today it would absolutely would have won.
Another thing is that the media available to rural vis-ŕ-vis urban populations – critically over the entire lifespan of most living at the time – was much more polarising in 2016 than it had been before the late 1970s.
During the Carter and Reagan eras, entertainment and music playlists (and perhaps opinions??) of the limited media available to rural people became much narrower than beforehand, and I have often felt that this difference may be a factor in increasing differentiation between urban and rural voters’ preferences, especially on social and racial issues. Lack of contact between the two had much deeper and long-term effects in 2016 than it could have in 1968, when a substantial number of poor white rural counties still powerfully backed Humphrey (and these would no doubt have gone more strongly still for a genuine peace candidate). I am not sure that expanding social media will solve the problem, because rural and urban people are unlikely to seek out media giving the other side’s viewpoint and most people have been shaped by the division in availability of entertainment (and likely opinions) since the Carter and Reagan Eras.