AbstractClassic theories of political ambition assume relatively stable and programmatic party systems. However, in many parts of the world, ‘fluid’ or ‘inchoate’ party systems do not provide ambitious legislators with the electoral benefits associated with stable party brands. In this paper, we examine ambition in fluid party systems: What is the frequency of party switching for legislators who seek re‐election, and what are their characteristics? What incentives do parties create to push away or retain legislators? And which parties do ambitious legislators seek out? Using a systems approach, we argue that in fluid party systems, legislators' office‐seeking behavior is driven by their attitudes towards parties and their constituents, their prospective evaluation of party performance, and ideology. Incentives for politicians to switch parties act as negative feedback mechanisms which keep the party system stuck in a fluid state. We test the determinants of party switching using two data sources from Ecuador: a public records dataset of every legislator who earned re‐election from 1979 to 2021, and the Parliamentary Elites in Latin America (PELA) surveys of all legislators from 1994 to 2017. Both sources distinguish between switchers and non‐switchers. The results support the conclusion that ambitious legislators in fluid party systems are strategic actors that seek or keep parties to maximize their probability of re‐election—and thereby further exacerbate the party system's fluidity.