Thanks for taking the time to thoroughly read these (and in more detail than I did! Lol, how embarrassing, especially #3! Good catch, and careless on my part. Sorry!)

Re #1 (the Atlantic), these passages capture most of my thoughts. And the other two articles touch on similar themes.

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> There is no obvious reason why a 19th-century movement led by Irish Catholic noneducated factory workers should become a 21st-century party for college grads, nonwhite voters, and software developers that defends gay rights, women’s rights, and legalized abortion. But it makes sense if you understand the Democratic Party through the lens of the modern city. Starting in the 1970s through today, Democrats and Republicans have been compelled to take sides on issues that hadn’t previously been politicized. And they have routinely sorted themselves along urban-rural lines, creating a pattern where there was once merely a tendency.

[...]

> As the writer and researcher Will Wilkinson argues, cities are magnets for individuals who score highly on “openness”—the Big Five personality trait that comprises curiosity, love of diversity, and open-mindedness.

[...]

> Urban residents trade cars for public transit, live in neighborhoods with local trash codes, and deal with planning commissions about shadows, ocean views, and parking rights. City residents are natural “externality pessimists,” to use Steve Randy Waldman’s clever phrase, who are exquisitely sensitive to the consequences of individual behavior in a dense place where one man’s action is another man’s nuisance. As a result, residents of dense cities tend to reject libertarianism as unacceptable chaos and instead agitate for wiser governance related to health care, housing policy, and climate change.

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I understand what you're saying, and it makes sense -- that (foreign) immigration was not the decisive factor of cities becoming more Democratic. I stand corrected. That's not just a technicality, it's a substantial correction. Thank you.

However, I do want to make the case that MIGRATION in general (of people, classes, workers, etc., both foreign but especially domestic, as you pointed out) shapes cultures in a way that ultimately helped further the urban-rural divide. Whether it's seeing other ethnicities, religions, cultures, education levels, or just economic classes and job types, the sheer density of city living forces people to learn to live together. Granted, it's not always easy easy, per the article's quote, "one man’s action is another man’s nuisance". But it happens.

And all these factored played together, weakly at first, and then more and more through the decades as party leadership polarized themselves to form stronger and stronger tribal identities... broadly, the Democrats taking the dense urbanites who favored openness (to cultures, religions, immigrants, etc.), protection through regulation, and education vs the Republicans who valued tradition, freedom (from other people and the central government, especially), and hard work that doesn't require higher education.

I don't think it has to have played out this way if not for the two-sided winner-take-all politics we have. But because we only had those two choices (for many decades), all those forces/themes from immigration to industrial policy virtuously/viciously reinforced each other. But I do stand corrected... foreign immigration was not the major input to those changes. Thank you for taking the time to really think through and discuss this!