I absolutely agree about the importance politics at local level. In fact, this is likely where regular citizens have biggest chance to actually influence an outcome.
I used to have a condescending attitude towards people who took sports 'way too seriously', but now I wonder if it is a net gain for society to give people a comparatively harmless outlet for these tendencies. Real harm is done when our policy discussions are dominated by the kind of tribalism, ideological intolerance, and rush-seeking engagement that seems to happen when people bring these tendencies to politics.
Tribalism in America goes deep, and you're right: rooting for a sports team is fairly harmless, and rooting for a political party is probably harmful, but I'd argue it's not nearly as harmful as as rooting blindly for an ideology. If you want your party to succeed, you should be engaged in your local politics, talk to people from the other side, listen honestly to their concerns, be willing to change your own opinion on specific policies, and push for those sensible policy changes to be adopted as part of your local party's platform. That's how you win people over, that's how you win elections, and how you enact real change that affects people's lives. Anything else is just yelling into your echo chamber, or getting into bar fights with the guys wearing the "wrong" jerseys.
Ideologies form the axes of socioeconomic space, they're not an ideal point, and the push toward an "us versus them" mentality in politics is embarrassing. It's a quirk that's arisen out of new media, an easily exploitable bug, and the sooner people see through the bullshit and we outgrow this, the better.