Arguing with certain doctrinaire liberals is very similar to arguing with creationists. Some liberals also have what I've taken to calling Holy Warrior syndrome. If you're fighting for a better world (whether in this life or the next), then your perceived opponent is evil, perhaps even Satanic. You, OTOH, are a white knight on a charger.
You are not having a debate in which you calmly explain what you think and why. It's war. Twist, shout, evade, distract! You're allowed to lie. (You may need this dispensation, as you must make no concession of any significance unto the hated foe.) You may dump great buckets of slop over your opponent at any time. After all, he's evil and all the good people on your side already know you're right. That's not even at issue.
It's the same disease (Holy Warrior Syndrome) with left and right reversed.