Honestly, we do have trouble with partisan writing, especially when it's our own side doing it. And that wasn't prejudice, it was a judgment, but it was based on what he has learned of you in posting with you over time.
Prejudice is when you immediately jump on someone cause they're a liberal, or a conservative, not when you make a judgment, right or wrong, about a person based on a great length of communication with them. It's still judging, it's just not prejudice.