No one is promulgating that. Very specifically, the idea forwarded is that both sides are human and therefore subject to human biases and groupthink.

That's not ideology. That's just well established fact. Also, don't conflate that with my description of the current political climate in the western world, which is largely one of polarization and tribal entrenchment.

The differences in degree matter a lot.

I'm glad you say that. You should go and look into the frequency of political violence, with an eye to finding things which are not reported by the media. A preponderance of incidents are committed by the extremist far left (Antifa &c) in 2018 and 2019. Then go and look into the frequency of people advocating for and tacitly approving of political violence. A preponderance of incidents are committed by the extremist far left and far right. Really, I'm tired of yahoos who basically say they can do anything, so long as it's no worse than the other side.

In terms of differences of degree, the extremes on both ends of the left/right spectrum seem to be trying for escape velocity from reason and sense.

Lately I've tried to avoid talking about -isms. Doesn't always work, because everybody talks about them, and I'm certainly deeply committed to a couple of -isms. But they tend to become both hollow labels and collections of dogma. "Socialism" is an obvious one that I often see abused. People oppose for example universal health care because they consider it socialist, and because socialist Russia/Venezuela was/is bad, universal health care must be bad. It's a stupid way to argue, but surprisingly common. The same is also true of "capitalism" of course. It can mean either the concentration of wealth in the hands of a small elite, or the free market. And even "free market" means different things to different people.

Better to let go of the dogmas and labels and talk about the results we want, while looking critically at what actually works and what doesn't.

Refusing to use names in this situation wont eliminate peoples fear, it will merely make it impossible to talk about what they are afraid of. Good sounding ideas can go real bad and it is good to learn from history. And I think that one of lessons of Russia history is that if people who are violent before gaining power or openly celebrate violence, they will be violent after.

Maybe it would be worth to talk also about Russia history and ideologies that affected it as they were.

Too much of the current left in the US celebrates harassment and violence. There's the bike lock bashing professor. There's the various hippies and other Berkeley residents interviewed by Tim Pool who give tacit support to Antifa violence. There's the simply-conservative father and son who were chased down by Antifa goons shouting epithets and given a beat down. There's the local news reporter who was bashed, his smartphone damaged for simply filming Antifa. There's Tim Pool himself, who is a Korean-American and center-left politically, being called "Alt-Right" so a mob of goons would come and beat him up. (And that's just off the top of my head. There's so much of this, and it's simply not covered by the mainstream media.)

How is it the left can object to violence done on its behalf overseas, but either tacitly support it or even perpetrate and celebrate it as a means of political intimidation? Something has gone corrupt.

Most importantly, USA is not 19 century Russia in pretty much any way. You went completely offtopic and also conflated antifa with social programs - in discussion about tribalism and biases. Achievement.

Part of that, particularly from Trump and his supporters, and probably from Antifa and similar groups, is excessive tribalism. They hate the other side and are eager to hurt them. Trump supporters are certainly quite open about that. Antifa might actually be different. It's also possible they actually believe that violence is necessary in order to stop fascism. But any support they may get from the left is probably pure tribalism again.

Of course there's also another aspect to violence from the right: things like lynchings in the 1950s, harassment of women, violence against Muslims and other minorities. Those stem from pure bigotry and a belief in (usually) white male superiority and a right to punish others if they seem too uppity or seem like a threat. I assume most people on the right abhor this, and yet some of these still happen.

This "right to punish others" should be a big red flag, be it from extremists on the right or the left. Both extremes have poor records, historically speaking.