So, if Aumann is to be believed, in those cases where we do talk enough, and in which we claim to share priors and fundamental values, disagreement is likely to turn nasty.

it's gotten a lot worse as far as I can tell in the past decade or so.

I agree it has gotten worse, though I would trace it back at least to the Bork nomination fight. So, if I want to stick to my AAT-based explanation of the facts, I need to claim either that we have only recently started claiming to have the same fundamental values, or that we are talking more.

I believe that there has been a convergence regarding claimed values, over that period, but the situation regarding communication is more complicated. Political activists (and they are exactly the people who have poisonous attitudes about the opposition) probably do communicate more, but they do so over completely distorted channels. Democrats learn about what Republicans are saying from the Daily Show, the Onion, and Pharyngula. Republican learn what Democrats are saying from Rush Limbaugh and Glenn Beck. I suppose the real question is why today's activists seem to think that these channels are sufficient.

Perhaps people would always have preferred those kinds of channels, but in the past they just weren't available.