Just to clarify: “political tribalism” is not simply “Tribe = Party” as you put it.
Mitt Romney, Donald Trump, and Mike Huckabee are all Republicans, but they are from very different tribes. Romney is from the genteel neoliberal economic wing of the Republican Party; Huckabee is pure social-conservative Southerner; and Donald Trump is an uncouth North-Easterner (not a typical tribal affiliation for a major Republican politician, obviously) who connects tribally with working- and middle-class Northern whites (despite the fact that economically he’s anything but working-class, his “style” still connects, hence why sometimes people say he’s a “blue-collar billionaire”). It’s impossible to mistake one for the other.
Tribe is more about culture, customs, manner of speaking, etc. Sometimes very different tribes are allied politically — what do dyed-in-the-wool God-n’-guns conservatives have in common with tech-geek Libertarian atheists? But both are Republicans (typically). This is very much a strategic alliance between the two.
On the Democratic side, a Northeastern socialist like Bernie Sanders has little, tribally, in common with a pure race-representative like Al Sharpton. My friends here at university have very little culture in common with someone like the Obamaphone Lady. But both vote Democrat.
One thing that shocked and horrified my friends about Trump’s candidacy was his fondness for his name in huge gold letters. This was every bit as frightening to them as anything that was in his policies. Why? Because my friends, culturally, are academic types and materially showing off in that way is anathema to the academic culture.
The tribal coalitions that are the “Democratic” and “Republican” parties do tend to stick together. Nonetheless, the vote is somewhat affected by what tribe a given candidate is from. Trump was, for largely tribal reasons, able to swing Michigan and Wisconsin voters that absolutely loathed Romney and wouldn’t have voted for, say, Jeb! or Rubio. Within the Democratic party, the primary between Hillary and Bernie split largely along tribal lines. Etc.
