Research on contemporary party organization emphasizes the growing power of the party in public office (PPO). The relationship between the external party organization and the PPO, however, is rarely analysed directly. We argue that the capacity of the extra-parliamentary party to regularly extract a fixed share from their MPs’ salaries – the collection of ‘party taxes’ or ‘tithes’ – can give us insights into the level and the nature of control the external organization exercises over MPs within and across organizational levels. Beyond providing new insights into an under-researched, but generally widespread, source of party funding across 25 parties in five Westminster democracies, the study shows the following: while party family shapes a party’s basic capacity to establish an intra-organizational rule that obliges MPs to regularly contribute parts of their salary, ideology drives the salary share a party can expect MPs to pay. The federal–unitary divide affects whether the national or the regional organizations are the main recipients of these contributions.