As grandes questões da politica a seguir pela União Europeia são decididas nas eleições nacionais. As eleições para o Parlamento Europeu acabam por ter relevância sobretudo para a politica interna de cada estado-membro.
The real effects of the European elections may be felt in London and Edinburgh rather than in Brussels (The Monkey Cage):
While the role of the European Parliament (EP) has grown significantly in the past decades, and particularly with the 2009 Lisbon Treaty, the European Parliament remains the weakest link in the E.U. power triangle. Although it has now become a co-lawmaker, the political agenda is still mainly set by the European Commission and approved by the European Council. Consequently, as the late Peter Mair already noted in a seminal article (pay wall) in 2000, the paradox of E.U. politics is that if you want to influence the European level, you have to vote in national elections. After all, it is at the national level that the members of the European Council and the European Commission are decided. In that sense, the 2013 German parliamentary elections were probably more important for E.U. politics than the 2014 European elections will be.
In a similar paradox, the importance of the European elections will not so much be at the European level, but at the national level of some E.U. member states. Journalists and politicians use the European elections often as a barometer of public opinion.