Some of it comes from union bosses, generously giving ACORN money taken from the dues of workers who might have preferred to keep it for their own needs. See this Washington Examiner piece.

Why do union bosses give money to ACORN? What does its “community activism” have to do with working people? Nothing. But ACORN helps elect friendly politicians through its voter initiatives, including many proven cases of fraud.

This is one more reason why we ought to junk the laws that promote compulsory unionism.