Socrates famously said that the one thing he knew was that he knew nothing. As it turns out, though, he was atypical. In general, people who are ignorant or incompetent tend to consider themselves very knowledgeable or competent, more so even than people who actually are highly knowledgeable or competent. Apparently this is because the skills you need to do something well are the same as the ones you need to recognize whether you are doing it well.
I have a strong suspicion that something similar is true when it comes to political bias and partisanship. Every once and a while you will meet someone who is hyper-partisan and biased, but who claims just to be an impartial rational observer of events. And you wonder: is he serious? Surely on some level he must recognize his own propensities, given that they are so glaringly obvious to everyone else. But no. From his perspective, it just so happens that all his political opponents are wicked and stupid. After all, didn’t he once criticize people on his own side for being too much like the other side? How can he be biased if he is critical of both sides? Etc.
When I hear criticism of a political figure whose politics I disagree with I try to do what I call a “tu quoque check.” Would I find this criticism persuasive if it was about someone I agreed with? The problem is that the brain is very crafty, and can come up with all sorts of pseudo-distinctions for how what the other guy did was totally okay when your guy did it.