But "politics is the mind-killer" is more about the tribal affiliation aspects of associating oneself with a party.

This is, in my opinion, a consequence of what I wrote above. You can't properly evaluate politicians and parties and you can't reliably predict their behavior, so you end up reacting emotionally and attaching yourself to one of the "teams". If you could reliably predict what a party will do if it wins the election, then you could evaluate and discuss the precise program without being "mind-killed". Since you generally cannot, you end up cheering for one of the team that you somewhat feel is more or less aligned with your position.