Party institutionalisation is a central problem in political science. The literature tends to understand it as a syndrome and therefore has difficulty explaining variations. This article suggests a new approach based on the transaction between a legislative party and its deputies, the failure of which is observable in party switching. Three routes to institutionalisation are identified by appealing to the vote-seeking, office-seeking or policy-seeking motivations of deputies. Poland has had a large volume of party switching, along with wide variation in the incentives facing differently-motivated deputies. Survival analyses of switching in four Polish parliaments find that vote-seeking is the most likely route to institutionalisation for Polish parties. Moreover, in this article a concrete hypothesis is established for comparative testing: legislative parties can survive as long as their popular support exceeds 40 per cent of their share in the previous election