From C.S. Lewis in Mere Christianity:

"Suppose one reads a story of filthy atrocities in the paper. Then suppose that something turns up suggesting that the story might not be quite true, or not quite so bad as it was made out. Is one's first feeling, 'Thank God, even they aren't quite so bad as that,' or is it a feeling of disappointment, and even a determination to cling to the first story for the sheer pleasure of thinking your enemies are as bad as possible? If it is the second then it is, I am afraid, the first step in a process which, if followed to the end, will make us into devils. You see, one is beginning to wish that black was a little blacker. If we give that wish its head, later on we shall wish to see grey as black, and then to see white itself as black. Finally we shall insist on seeing everything — God and our friends and ourselves included — as bad, and not be able to stop doing it: we shall be fixed for ever in a universe of pure hatred."

The liberal Washington Monthly blogger who brings us this quote:

If you give in to "the sheer pleasure of thinking your enemies are as bad as possible", it's easy to see how you could end up thinking things about them that it is implausible to think about any group of human beings.. Your opponents become cartoons in your mind, and the normal duty to be charitable and generous, or even realistic, in your views about other people seem not to apply to them. You stop thinking of them as fellow human beings, and start thinking of them as enemies...

No one — not liberals, not conservatives — should forget that their opponents are human beings. And no one can afford to start down the road Lewis describes, in which you allow yourself to be disappointed when your opponents aren't as bad as you first thought, or want them to be as bad as possible. And no one should get so wrapped up in political fights that in focussing on the mote in someone else's eye, they lose sight of the beam in their own.