“I don’t have any disagreement whatsoever in what you’re describing; I simply think that “tribe” is an imperfect word to describe it.”

You are of course technically correct that Tribe used to describe the Social Unit that lay above Family but below Nation, but there are two problems with your stance:

1. Tribe is now in common use as a term for groups with common beliefs who act in a tribal way towards outsiders. Common usage in the end will define the new meaning of the term, unless there are enough people like you who push back to the original meaning of Septs or Clans.
2. It is hard to deny that Democrats and Republicans behave towards each other as if they were two Tribes about to go to War. “It’s something a lot thicker than political/cultural opinions, which are somewhat fluid.” I have tried to explain to Democrats how reasonable, tolerant people could vote Republican, but have found their political and cultural opinions to be anything but fluid. What is more, the behaviour of American political supporters continues to grow more tribal every year. I was appalled by the way GW Bush was treated by the mass media during the election campaign for his second term in office, but that pales into insignificance in comparison to the outright hatred directed at President Trump.

The other point that you quoted, “the phrase “Merry Christmas” was actually deeply important to them because they’d grown up with it, and that hearing it replaced by “Happy Holidays” was actually a significant, meaningful change,” actually understates it. It implies that it was a meaningless tradition that they missed for sentimental reasons. In order to avoid supposed offense to Muslims, Hindus and Jews, Christians were banned from the use of the traditional benediction “Merry Christmas”, a core part of their faith. I first heard “Happy Holidays” on a visit to Seattle in 2004, and it grated on my ears. No other country has taken up this idiocy, and I fail to understand how a supposedly Christian Nation like the USA could allow this attack on their religious freedoms to go unopposed. No Muslims, Hindus or Jews, to my knowledge, have ever been offended by the words “Merry Christmas.” I wish my Hindu friends “Happy Diwali” at their Festival of Light, and am not offended when I receive the blessing in return: it is all about the spirit in which it is intended. When I asked a muslim friend about it, he was contemptuous of so-called Christians abandoning their faith for political correctness. ‘Imagine if you banned Muslims from saying “Salaam”’ he said: “There would be an uproar!”

And rightly so.