Party institutionalisation is a central problem in political science. The literature tends to understand it as a syndrome, and therefore has difficulty explaining variations in institutionalisation. We suggest a new approach based on the transaction between a legislative party and its deputies, the failure of which is observable in party switching. We identify three routes to institutionalisation 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. Our survival analyses of switching in four Polish parliaments find that vote-seeking is the most likely route to institutionalisation for Polish parties. Moreover, we establish a concrete hypothesis for comparative testing: legislative parties can survive as long as their popular support exceeds forty per cent of their share in the previous election.