When it comes to political polarization, it's confirmation bias all the way down.

By Ronald Bailey @ Reason.com and in the January 2022 issue

[....] Today, if you are a member of one of the two major American political parties, you are statistically likely to dislike and distrust members of the other party. While your affection for your own party has not grown in recent years, your distaste for the other party has intensified. You distrust news sources preferred by the other side.  Its supporters seem increasingly alien to you: different not just in partisan affiliation but in social, cultural, economic, and even racial characteristics. You may even consider them subhuman in some respects.

You're also likely to be wrong about the characteristics of members of the other party, about what they actually believe, and even about their views of you. But you are trapped in a partisan prison by the psychological effects of confirmation bias. Being confronted with factual information that contradicts your previously held views does not change them, and it may even reinforce them. Vilification of the other party perversely leads partisans to behave in precisely the norm-violating and game-rigging ways they fear their opponents will. It's a classic vicious cycle, and it's accelerating [....]