Read this and answer the question. If the question is unanswerable, say "unanswerable".

Political parties, still called factions by some, especially those in the governmental apparatus, are lobbied vigorously by organizations, businesses and special interest groups such as trade unions. Money and gifts-in-kind to a party, or its leading members, may be offered as incentives. Such donations are the traditional source of funding for all right-of-centre cadre parties. Starting in the late 19th century these parties were opposed by the newly founded left-of-centre workers' parties. They started a new party type, the mass membership party, and a new source of political fundraising, membership dues.

What do special interest groups receive for paying membership dues?
unanswerable