In territorial politics, political parties are key actors – be it state-wide or regionalist parties – for constitutional reforms underlying regionalisation or federalisation processes. From the agenda-setting to the implementation of the reforms, they aggregate and catalyse territorial/national interest in order (not) to enable institutional reforms. In most representative democracies under such regionalist pressures, it is however individual MPs that ultimately vote the constitutional reforms. Of course, party discipline strongly constrains MPs’ freedom and frames parliamentary behaviour. The literature thus often explores territorial politics from a macro-perspective – political parties’ positions – rather than at a micro level – individual MPs. Yet, exploring MPs’ views on constitutional reforms bring interesting perspectives on intra-party dynamics complementing inter-party approaches. What explains MPs’ preferences on constitutional reforms? To explore this question, we surveyed every representative of Belgium’s legislative assemblies with a standardized questionnaire in the middle of the longest period without federal government (Summer 2011). We hypothesize that ethno-linguistic identity and party affiliation are strong variables explaining MPs’ preferences but that candidates’ attributes – be it (multi-level) former political experience, gender, political generation – should also be taken into account. Based on the Belgian case study, this research will help us apprehend the rationale behind political elites’ attitudes negotiating the new boundaries of the state and above all shed light on territorial politics in Europe.