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> the shift in the basic, underlying "we're-all-in-this-together" ethos has been not just noticeable, but profound

Depends on your value of "we."

There is certainly lower solidarity/interest in public life among the straight white landowning Christian men from good families who used to participate in things like freemasonry and Rotary.

On the other hand, both the left and right have been adding previously marginalized demographics to their constituencies and platforms.

I don't think there's any less solidarity going around. My intuition is that more people than ever have their voices heard and interests represented in public life. But rather than one line of solidarity across the center of the political spectrum, we now have two disjoint lines reaching from each party's center to what used to be its marginalized fringes. (Not just in ideology, but in identity).

We also just aren't in this together so much anymore. The rise of coastal cities means the country is increasingly split between (at least) two totally distinct patterns of geography, economics, culture, built environment, and lifestyle. The correlation between population density and vote share on the latest electoral map is staggering.