I had a professor last semester who would often find excuses to go off on a tangent and give a ten-minute lecture on how the "collective human animal" needs to stop beating itself up and we must all come together as one in peace and love and harmony. This was often followed immediately by a ten-minute tirade about how any politician he disagreed with (e.g., Bush, Cheney, Lieberman) was a "treasonous, Jello-spined *******" who should be assassinated. He never saw the contradiction in effectively saying "Everyone needs to love each other, and if you're part of the 50% of the country that's not loving enough, I hate you."
If you only do good to those who do good to you, what good is that? Even the tax collectors do that. What ****ed me off so much about that guy was that he was doing the same thing everyone does -- the bare minimum of loving only the people who love him -- but he thought that because he was doing it, it made him morally superior. You guys are evil because you hate me, and I hate you for that, which is okay for me to do because you're evil.
So yeah, [people who politically identify as] liberals can definitely be closed-minded, just like the rest of us. I would venture to say that even most people who seem open-minded tend to just be open-minded to people who are open-minded the same way they are. And everyone is that way. But the ones who do it and call it open-mindedness get societal brownie points for being the right kind of closed-minded.