The common denominator here is dependency.
Blacks, single women, unions, young people, and illegals...
The left offers the blacks and illegals welfare that helps in the short term, but getting a job takes those benefits away.
With single women and unions, the promise is that if you vote for them, the law will be changed to give you favor in the workplace and added benefits that are strong-armed from employers.
With younger voters since the economy tanked, they're hopeful that their student loans will be forgiven.
Like Alfonzo Rachel says. Start being dependent on someone, and you're in their pocket.