One of the fundamental issues is that some people so strongly tie their identity to a party that an idea can by definition not be good if the other side supports it. It's not a project to improve society, it's a game, a war, and the other side is evil and must be defeated.

This may be true in some cases, but when you have a system that entrenches two major parties, you don't get to vote for idea X, you have to vote for party A or party B. If you have some policies that are dealbreakers, and party A agrees with you and party B doesn't, you can't vote for party B even if they support idea X. So then you become a supposed "party B fanboy"