Democrats and "Liberals" tend to be angry people. The anger tends to be free-floating and unfocused and probably results from some psychological dynamic that has nothing to do with anything they blame it on. They use this anger effectively by attributing it to some high-minded outrage, such as outrage at injustice or the way some other politician has handled a situation--something that has nothing to do with the real, basic cause of the anger.
They claim that their anger is fury at something that needs correction, whereas, in truth, they are just angry people looking for something to be angry about.
Some of them actually feign anger.
Sometimes they're angry because of a bad self-concept, which may be richly deserved. The pain of self-contempt is transmuted into anger, which is easier to accept than an admission of the conviction that one is in fact a miserable, contemptable wretch. In other words, the anger is misdirected self-contempt.
Sometimes it's as simple as, "I'm angry because I'm about to be defeated, but I'm going to claim to be angry because of the President's policies; at least then I won't look as silly as if I told the truth about why I'm so angry."
Don't you get tired of all this anger? Angry people get tiresome fast.
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.