The first defense is that the bylaws of the DNC are the DNC's internal affair not enforceable by a court of law. But an association's bylaws constitute a contract between its members and is enforceable as are all written contracts in a court of law.
Just my laymen’s reading.
Are the plaintiffs members of the DNC (national) or members of a state-level group?
Giving money to the national DNC does not generate a membership card.
I do not think this fishing expedition has a net that will catch anything.