A new method is proposed for the apportionment of seats in the European Parliament among political parties at European level: compositional proportionality. Compositional proportionality achieves two goals. Firstly, it safeguards the composition of the European Parliament, that is, it realizes the preordained allocation of seats between the Member States. Secondly, it apportions the subset of seats to which it applies proportionally to unionwide vote totals, that is, it reflects the political division of the Union citizens according to the motto “one person–one vote”. Compositional proportionality is demonstrated using the data of the 2014 elections. However, since past elections were contested by domestic parties rather than by European parties, the unionwide vote totals of those domestic parties who joined the same political group in the 2014 European Parliament are hypothetically substituted for the non-existing vote totals of political parties at European level.A new method is proposed for the apportionment of seats in the European Parliament among political parties at European level: compositional proportionality. Compositional proportionality achieves two goals. Firstly, it safeguards the composition of the European Parliament, that is, it realizes the preordained allocation of seats between the Member States. Secondly, it apportions the subset of seats to which it applies proportionally to unionwide vote totals, that is, it reflects the political division of the Union citizens according to the motto “one person–one vote”. Compositional proportionality is demonstrated using the data of the 2014 elections. However, since past elections were contested by domestic parties rather than by European parties, the unionwide vote totals of those domestic parties who joined the same political group in the 2014 European Parliament are hypothetically substituted for the non-existing vote totals of political parties at European level
