why we're polarized

Klein argues that although polarization has affected both sides, the Democrats have managed to muddle on, but the Republicans have gone completely off the rails.

Stop for a second before reacting here. I get the impression that Klein understands he is taking a risk (not an actual risk of decreased popularity, given the givens, but some kind of metaphysical risk to his soul) by abandoning his previous attempt at a neutral stance and coming out like this. I think he feels bad about it, and that he considered not writing this chapter on that basis. I think it's very important to him that we consider the possibility that he wants to be neutral, is trying as hard as he can to be neutral, but that even from an attempted-neutral point of view he thinks the decline of the Republican Party is a threat to the stability of the country. And I think it's very important that we maintain a stance where we recognize this is a potentially true state of affairs - it really is possible that one party is much worse than the other! - and don't automatically condemn Klein for raising the possibility.

That having been said, I don't think he acquits himself well here. Some of his arguments aren’t great (the Republicans "launched a bizarre and unpopular campaign to impeach Clinton", but Clinton was obviously okay and didn't deserve impeachment, so the GOP has gone crazy and is a threat to democracy). He refers to data "showing Congressional Republicans have moved further left than Democrats have moved right", which I think is a typo (isn't the usual argument that Republicans have moved further right than Democrats have moved left?) but he never gets around to presenting it, instead gesturing at how it's obvious by looking at the Trump campaign vs. the Clinton campaign. This does not seem obvious to me. Trump holds basically the same positions that Americans in the mainstream of either party would have held in a less polarized time (eg 1995); Clinton holds positions that everyone in 1995 (including her husband) would have thought insane, radical, and ultra-far-left.
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(my own model here is that the social justice movement pioneered a much angrier, more radical, more in-your-face style of identity politics, and the conservative movement scrambled to figure out how to fight fire with fire despite a big handicap - conservative identity politics based on white, male, etc identity are understandably taboo. They flailed around for a while, mostly failed, and are now experimenting with how far anger can go as a substitute for a coherent philosophy. Needless to say, this isn’t how Klein thinks about any this.)

Here.