I will be on vacation when this post appears, so it seems appropriate to take a break from facts and engage in speculation.
"Tribalism" has become a favorite word in writing about contemporary politics. It seems like the wrong word to me--the key thing about a tribe is that you don't choose it, you are born into it. A second feature is that tribal leaders have a good deal of freedom in conducting relations with other tribes (see this paper, p. 141)--if they say that we've traditionally been allied with group A, but now it's in our interests to make an alliance with group B, the members will go along. Tribal politics can involve intense conflict, but it can also involve toleration and coexistence--you can't blame someone for being born a member of a different tribe, and there's a chance of winning them over by making a deal with their leaders.
What we have now is ideological politics. where people choose a side because it represents the right principles. Ideological politics necessarily involves conflict. You can definitely blame someone for choosing the wrong principles; also, leaders have less freedom, because the members may revolt if they seem to betray those principles. It's sometimes said that Republicans have abandoned their principles to follow Trump, but when those principles are specified they turn out to be things like free trade, concern about budget deficits, and the rule of law, which aren't traditional Republican or conservative principles--they cut across party and ideological lines, and are probably strongest in the "good government" center. If Trump did something that really went against conservative principles--e.g., proposed a program of infrastructure spending financed by closing tax loopholes that benefit high earners--there would be a revolt. Of course, I can't give evidence of that, because Trump has conformed to conservative orthodoxy on everything that's important to conservatives--you don't have to take my word for it, you can take Mitch McConnell's.
That raises a question of why ideological politics grew in the United States. In almost all other affluent democracies, it has been declining for a long time, and the decline seems to be continuing. I will turn to that in my next post.