word for racial polarization (similar to "partisanship")

Solution 1:

A word that covers stratification according to some group identity is tribalism.

Tribalism: strong loyalty to one's own tribe, party, or group.

This term is broader than what you're asking for, insofar as it covers strongly identifying with any kind of group, whether it be a race, creed, favourite hockey team, etc. In order to make your point clearly you might have to use the phrase: racial tribalism.

Solution 2:

While it's not a single word answer, identity politics might be related:

Identity politics are political arguments that focus upon the self-interest and perspectives of self-identified social interest groups and ways in which people's politics may be shaped by aspects of their identity through race, class, religion, gender, sexual orientation or traditional dominance.

Solution 3:

I think a good fit for your description would be ethnocentrism.

According to American Heritage Dictionary, it can be

1. Belief in the superiority of one's own ethnic group.

2. Overriding concern with race.

(Clearly we'd be going with sense 2 here.) Vocabulary.com indicates that it is a sub-type of partisanship.