ABSTRACT
Why do legislators sometimes challenge the party leaders and vote against the offi cial party line? When do some legislators who vote with the government strategically challenge their party on selected votes only, while others opt for outright (public) party defections? The occurrence of party defections and party dissidences is critical to understand the dynamics of coalition-making: these events effectively disrupt the electoral link between voters’ policy preferences and their representative’s actions, bring uncertainty to the composition and durability of congressional majorities, and alter the structure of the party system as a whole (Mainwaring 1999:146; see also Linz and Valenzuela 1994; Jones 1995; Mainwaring and Shugart 1997; Amorim Neto 1998b).
