But indeed, they do have a common thread: envy. Whether or not it's even real, Obama and the left have forged the perception of disparity between classes, races and genders, and carefully fostered a seething rage towards the perceived "haves". This is their common thread; this is what binds his followers. It allows them to blame the world around them for their misfortune, and alleviates them from all responsibility for their present condition. Contrived as it is, it takes no effort to glom onto this line of thinking.
Envy is certainly a common (but vague) element but so is hunger and thirst.
What Obama has done (as any Alinsky trained community organizer is programmed to do) is identify the unique desires of each group and make an appeal to that group.
Doing that in an aggregate, collective fashion should weaken or dilute each individual group appeal, but somehow Obama has overcome this problem.