Political parties are key actors in federal systems to ensure coordination among the different layers of government. In countries under strong regionalist demands, they play a particularly significant role in balancing claims for greater regional autonomy while preserving the state integrity. The literature on party politics as well as on regional and federal studies have challenged the assumption of parties as monolithic actors with homogenous constitutional preferences. In this wake, this contribution explains the degree of preferences heterogeneity amongst Belgian parliamentarians. Based on an original survey of regional and federal MPs carried in 2014-2015 (277 MPs, i.e. 62 percent of response rate), we show that intra-party variance exceeds inter-party variance. To explain that, this contribution tests two main hypotheses: (1) MPs’ constitutional preferences are explained by individual attributes (gender, identity, career pattern) and (2) MPs’ constitutional preferences are explained by institutional parliamentary cleavage (the type of assembly – regional or national – in which MPs served). Overall, the paper contributes to ongoing theoretical debates about the role of parties in constitutional reforms in parliamentary federal systems.
