Matthew Continetti has the story.
When things aren't going his way, the Angry White Liberal wails and gnashes his teeth, rends his garments, and hurls invective at the opposition. His rhetoric and prose is so heated, it's gotten to the point where you need to put on oven mitts before opening the paper. He is so convinced of the righteousness of his positions that he lashes out uncontrollably at anybody who disagrees with him. For the Angry White Liberal, dissent is anathema. Antagonism is illegitimate. Only conformity to prevailing liberal opinion is enough to still his rage.
Yeah, I run into them all the time. Disagree with any minute detail of whatever the current Holy "Progressive" Dogma and Doctrine is, and you are automatically a far-right-wing Republican* racist brown-shirt sockpuppet of BusHitler ... or whatever the demon of choice is nowadays.
[* -- I'm a pro-choice pro-2nd-Amendment pro-gay-rights pro-reality centrist pluralist currently registered as a Democrat. To the deranged True Believer "progressive" liberal that apparently make me a far-right-wing Republican. Or maybe just disagreeing with them in any tiny detail does. Don't ask it to make sense.]