What? You have the Bernie socialists vs the Clinton centrists on one side, and the Trump MAGA warbirds vs the McCain moderates on the other. It sure looks fractured to me.
https://fivethirtyeight.com/features/the-two-cracks-in-the-r...
http://www.pewresearch.org/fact-tank/2014/06/12/polarized-po...
https://twitter.com/gelliottmorris/status/888904322326638592
Liberals are at 0.38, and the most extreme they've ever been was 0.39. They have been somewhat more liberal than they were in the 40s, but they've generally been within the same band of scores for the last 100+ years.
Conservatives, on the other hand, are at 0.49, which is the most conservative they've been in the last 200 years. They've basically doubled since the 1970s.
So, objectively, conservatives are more extreme (0.49) than liberals (0.38) today. Also, the most extreme conservatives are more extreme than the most extreme liberals.
https://en.wikipedia.org/wiki/NOMINATE_(scaling_method)
I think this may be the first time I've seen this, but I'm quite confused about the claim to be able to make comparisons across time by simple statistical methods. The model has a clear statistical meaning about the similarity of different members of the same Congress, who have all voted on the same set of questions. But surely the issues that the Congress is actually voting on are radically different from decade to decade? Slavery, the Federal income tax, alcohol prohibition, the New Deal?
Meanwhile, thus far the "farther-left (and maybe even insurgent) Democratic wing" for all the noise it seems to make has had little effect on the party. They aren't electing people, they aren't even really challenging incumbents very much.
Contrast this to the steady rightward march of the Republicans over the last 30 years: the rise of the Evangelicals in the 70s, Gingrich's 1994 "Republican Revolution", and more recently the Tea Party wave of 2010 and the explosion of an apparent Trump faction. All of these lead to not only actual electoral victories, if not dominance in the Republican Party in some cases, for the more extreme factions, but were coupled with real threats of incumbents who didn't play ball being "primaried" by more extreme challengers.
As I said, it's possible that the surprising performance of Sanders and the popularity of people like Warren may be the beginning of similar change in the Democratic Party, but until that actually happens it's just not comparable to what's happened to the Republicans.