[x]

Bear in mind that Warren was a Southerner (though in later years, he lived mostly in Connecticut and Vermont), so he’s giving a Southern view (though he had become critical of segregation by the time he wrote) — and that was how many Southerners viewed, and some still view, Northerners (especially New Englanders). It was in large part an “eye of the beholder” perception, but it was encouraged by some historians of his day, who blamed the “self-righteous” abolitionists for opening a national wound (slavery) that they assumed would somehow have healed itself.

Warren was also thinking about our national habit of going on crusades abroad. He had the idea that the Northern belief in our national righteousness and innocence led us to view our interventions abroad as moral crusades. I’m not sure that actually works. At least one big war was forced on us. All countries assume that their cause is just when they go to war. Southern presidents (or presidents with massive Southern support) led us into some of those crusades. Still, Warren’s reflections are an interesting example of how one Southerner who was much closer to the war than any of us are — actual, living Civil War veterans and widows in the family and neighborhood — saw things.

[BroJoeK]

x: "Bear in mind that Warren was a Southerner (though in later years, he lived mostly in Connecticut and Vermont), so he’s giving a Southern view "

I'm not disputing that "Northern self-righteousness" is what Southern Democrats said and felt, no doubt they did.
I am saying it's equivalent to the accusation which became popular in the 1980s, that Republicans are "mean-spirited", what does that even mean?
It means nothing except: you disagree with Democrats on something important, so you are, "self-righteous", or "mean-spirited", or "racist" or even "fascist", etc.
These are ad hominem attacks intended to shut down disagreement and leave Republicans sputtering and ineffective.

Sure, you can easily argue that "Republicans do it too" and cite decades of claims that Democrats were "soft on Communism", if not outright Communists themselves, certainly they were Socialist-friendly.
And in response, for decades Democrat media ran campaigns against "McCarthyism" and "Hollywood blacklists", but we don't see that so much anymore because today even real Communists seemingly rejected communism, while nobody is ashamed of the word "socialist" anymore.
So, are there any pejoratives Republicans have used that are as remotely effective as the many accusations hurled by Democrats against us?
None that I know of.

Bottom line is: I see no reason to take any such Democrat accusations seriously or to even attempt defining objectively what they might mean or figure out some strategy for responding & countering them.
The only real response I see is pointing out how unfair or meaningless those accusations are and hopefully winning over enough voters to make them also irrelevant.