To sustain an out-group/in-group dynamic, each electoral cycle brought a "more conservative than thou" purification to win ratings, each new primary challenger denouncing their "RINO" predecessor in a surprisingly rapid spiral. This recalled the "eat their own" spiral taken by the French Revolution and, to a certain extent the early Bolsheviks. So there was in each case an element of populism and crowd psychology as well. I apologize this isn't very well framed, but I'm try to define the dynamic and not sure how to distinguish it from the sort of party power struggle you describe.

@NelsonAlexander: the exact same GOP dynamic you are describing applies to Dems: they are getting more and more "progressive". This has nothing to do with "revolution" and everything to do with the election system of "winner takes all" which leads to a 2-party system and the within-party primary/caucus system that mobilizes the faithful.

@NelsonAlexander denouncing someone else isn’t really the same as cutting their head off. Just a thought.

@sds The question for "more and more progressive" though is whether it's internal within the Dems or whether it's the Dems mirroring best-practise equality principles as set by law and by public opinion. Granted that this will vary from area to area, but the majority no longer believe that Jim Crow was right, mixed marriages are a sin, or that gay people should be persecuted. This represents a general shift in society over the last 60-some years, and it seems plausible that the Dems are simply following the electorate. Conversely people who agree 100% with Trump are very much a minority.

Really? The only exception to post-revolutionary bloodshed is America? What about the anti-Communist revolutions in Poland, Germany, Czechoslovakia and Hungary in the 1990s? What about the anti-Fascist revolutions in Spain and Portugal in the 1970s?

@NeMo: yes, those were not violent revolutions created by "professional revolutionaries". They are out of scope.

It's for the querent to say what's in scope. Your distinction of 'professional revolutionaries' doesn't make any sense at all. The French Revolution wasn't led by 'professionals' who set out to create a republic - it was one thing happening after another. What about the downfall of Ceaucescu in Romania? Or collapse of apartheid in South Africa? Neither of them led to Stalinesque purges, even if both countries were left with very serious problems. I'm afraid this answer smacks of sentimental exceptionalism, not real analysis.

In line with your suggestion to OP I've removed the references to the modern GOP from the question and your answer. If you don't like that, you can always put it back.

Yes, "professional revolutionaries" does limit it largely to the 19th century marxists. The original dynamic I am trying to compare historically was based on the rise of right-wing talk radio. In this "oratory" the need to push ratings by outraging listeners produced a feedback cycle that toppled and replaced primary candidates of ever greater extremism, the 2000s spiral of "RINO" hunting and denouncing. It appeared so nearly "mechanical" or "biological" I was wondering how such tribalist friend/foe oratory works. Yes, there's lots of polarization, but really nothing like it among Dems.

@NelsonAlexander: I think your political preferences affect your perception of polarization.

That's probably inevitable, but the rise of talk radio on the AM dial and its enormous impact on GOP primary candidates is pretty much a matter of record and that fact that there has been nothing comparable with the Dems seems to enjoy broad agreement. Whether or not you think that's good thing is where the political preferences come in. But I agree this is not the forum for this topic, so I'll give myself a century or so as a statute of limits. And I'll revisit my question in a revised form.

@NelsonAlexander: There is nothing even remotely similar to "the squad" on the republican side. 'nuff said.

@NelsonAlexander: as for revising the question, it is an extremely bad form to revise a question that has answers. Better ask a new one.

Yes, I meant a new, more focussed version of the question.

@sds. Certainly there is no comparing Stalinism with American Revolution. But Stalin's purges were nearly a generation after the fall of the Czar, though they are still the outstanding examples of "party purge." And we should not forget that American Revolution was followed by purges and expatriation of loyalists, scattered Indian wars, and the power consolidations of Shay's and Whiskey rebellions. This is all in a pre-modern scale, of course, but not insignificant. The "Founding Fathers" were at one point the more extreme faction among "moderates" in a roughly divided colony.

The side that wins the post-revolutionary power struggle is the clique with the most tactical skill, not the "moderates" or "extremists". Interesting point of view! Of course, ever tightening the definition of what it takes to be an ally of the revolution, and casting the views of your opponents (especially nuanced or merciful ones) as solidarity with the "other side", is a very effective tactic; and would tend to push the movement toward extremist - or at least extreme - interpretations and acts. Revolutions are characterised by ever increasing frenzies of denunciation and lynching.

@'Nuff said? Hardly. I can think of a dozen more things to say on the subject, which would prove you wrong. But I don't particularly want to say them, you definitely don't want to hear them, and they are not a fit subject for a history site. That's why I'm flagging comments relating to modern politics to be removed.

@NeMo: I hope the moderators have enough sense to abstain from removing upvoted comments.

If gamification was enough on its own, we wouldn't need moderators at all.