Whenever I read rhetoric like this, I really want the author to make a strong case for this actually being a new phenomenon... that twenty or fifty or a hundred or three hundred or however many years ago this "age" we're asserted to currently be in supposedly started, we didn't imagine that our political opponents were evil and their beliefs the result of various ethical flaws.
Does Bottum?
I strongly doubt it. Mainly because many of the things we recognize and revile in today's political discourse had a thriving existence around the turn of the century: Yellow Journalism, political machines, Anarchists around every corner... And it's hard to argue that, say, the 70's were a more moderated time.
It is possible that specific metaphorical frameworks and memes are more recent and promoted through a greater volume of media than back then, but there is a far greater volume of media in general.
One of my favourite Less Wrong articles is Politics is the mindkiller. Part of the reason that political discussion so bad is the poor incentives - if you have little chance to change the outcome, then there is little reason to strive for truth or accuracy - but a large part of the reason is our pre-political attitudes and dispositions. I don't mean to suggest that there is a neat divide; clearly, there is a reflexive relation between the incentives within political discussion and our view of the appropriate purpose and scope of politics. Nevertheless, I think it's a useful distinction to make, and so I applaud the fact that Eliezer doesn't start his essays on the subject by talking about incentives, feedback or rational irrationality - instead he starts with the fact that our approach to politics is instinctively tribal.
This brings me to Joseph Bottum's excellent recent article in The American, The Post-Protestant Ethic and Spirit of America. This charts what he sees as the tribal changes within America that have shaped current attitudes to politics. I think it's best seen in conjunction with Arnold Kling's excellent The Three Languages of Politics; while Kling talks about the political language and rhetoric of modern American political groupings, Bottum's essay is more about the social changes that have led to these kinds of language and rhetoric.
Video of a related lecture can also be found here.