During his successful 2005 campaign to be elected Leader of the Conservative Party, Cameron pledged that the Conservative Party's Members of the European Parliament would leave the European People's Party group, which had a "federalist" approach to the European Union.
În timpul campaniei pentru conducerea Partidului Conservator, Cameron s-a angajat că sub conducerea sa, europarlamentarii partidului se vor retrage din Partidul Popular European, care avea o abordare „federalistă” asupra Uniunii Europene.