Impact of Western Balkan Accession on the European Parliament: Projecting Seat Allocation and Power Dynamics

The European Parliament (EP) has long positioned itself as one of the most vocal advocates of EU enlargement. Although its official role remains limited throughout the accession process, members of this institution play an important role in shaping the enlargement debate, informing EU citizens, and influencing decision-makers in their home countries. Moreover, the EP has also formally supported gradual institutional integration – an idea introduced by the Staged Accession Model1 – favouring the merit-based and progressive inclusion of candidate countries in the Union’s institutions, including the Parliament itself, prior to membership.2 While it continues to be a champion of enlargement – framing it as a means of strengthening the EU’s credibility and increasing its geopolitical weight – its 2025 Resolution on institutional consequences of the enlargement negotiations highlighted the necessity to “address the implications of enlargement for the composition of Parliament” to ensure EU’s efficiency as it enlarges.3 With several candidate countries on track to acquire membership in coming years, the questions of institutional consequences of enlargement now warrant more systematic scrutiny – not just regarding the impact on the redistribution of seats but also on the balance of power between parliamentary groups. Providing answers to these questions is especially relevant in an era of increasingly fragmented majorities, where even modest changes can influence legislative bargaining and voting outcomes.

With such pertinent questions at hand, this paper examines how the hypothetical concurrent accession of all Western Balkan countries (WB6) would shape the EP. First, it estimates the implications for the allocation of seats by simulating two scenarios: 1) a short-term scenario in which temporary derogations from the treaty-based numerical cap are used to ensure immediate representation during the ongoing parliamentary term, and 2) a longer-term scenario in which seats are reallocated within existing treaty limits for the subsequent legislature. Second, the paper assesses the potential impact on parliamentary power dynamics if enlargement were to happen during the current term. It does so by mapping national party representation onto European political groups and proportionally translating domestic parliamentary strength into hypothetical EP seat distributions. Overall, the paper argues that, given the relatively small population size of the WB6, their accession would require only limited institutional adjustment of the EP’s composition while leading to a moderate reconfiguration of the EP’s political balance. Acknowledging that stable majorities across EP dossiers are becoming increasingly difficult to sustain, the paper further suggests that the gradual involvement of candidate-country parliamentarians in the EP’s work could help support its post-enlargement cohesion and effectiveness.