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.

This is a "both sides are the same" fallacy. Just because it looks fractured doesn't mean it is equally so:

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

Agree. Also, see the DW Nominate scores for congress. https://voteview.com/parties/all

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.

This method is explained a little bit at

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?

Most Trump supporters would suggest they are the moderates when it comes to military intervention, and the McCain traditionalists are the hawks.

Absolutely. Bannon was an anti-interventionist force within the campaign/admin. But he's gone now, so we might see the WH turn towards neo-conservatism.

To add to the other comments, it's a false equivalence because, while there certainly are factions within both parties, they aren't equally divided. For example, there's simply nothing comparable, in terms of power, effect, and influence, to the Tea Party/"Freedom Caucus" of the Republicans on the Democrat side, at least not yet. Sanders or Warren might prefigure a major shift or break in the party, but it hasn't happened yet. On the Republican side it has happened several times in the past 10 years, first with the "Tea Party" wave and now with Trump.

Doesn't the amount of votes that Sanders got in the primary indicate that there is a farther-left (and maybe even insurgent) Democratic wing that has at least as much "power, effect, and influence" as the Tea Party? Or do you need to see it actually unseat senators in primaries, instead of almost unseat presidential candidates?

Yes, we'd generally want to see evidence of something before talking about whether it exists. The Tea Party unseated incumbent Senators and Congressmen and forced (or enforced) policy changes in the Republican Party. While they have fallen from their 2010 height, they still wield some power, especially in the House via the "Freedom Caucus".

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.