Taranto nails it. Read it. All. Hard to pull part for quotation because it's all good.
And the sum:
This column has argued that the Obama presidency poses a deep political and psychological challenge to the Democratic left, which relies on the perception that racism remains prevalent in America, and that the GOP is racist, both to motivate black voters and to maintain its own self-identity as morally superior. That explains the desperate need to stereotype Obama critics, and conservatives more generally, as racist.
I would add that another big factor, other than motivating black voters, is to smear the GOP so badly that voters on the fence who are convinced that the liberal 'blue' model is broken won't be comfortable voting for candidates that have been depicted as evil haters.