Envy and resentment are probably at the top of the list. A national party must of necessity be a broad coalition, but resentment probably comes closer to being a common denominator for the dems than any other single factor.
Sadism is a big component of "activism" and the "Liberal" mindset. Resentment, envy, righteous indignation--all are frequent manifestations of deep-seated, free-floating anger, often misdirected self-contempt--and all can find expression or release in sadism.