While he was studying in Bern at the end of the 1940s, Eppler got to know Gustav Heinemann, one of the founders of the Christian Democratic Union (CDU). Heinemann became Minister of the Interior from 1949 to 1950, but then left the cabinet, and later the CDU, together with several other party members who disagreed with Chancellor Konrad Adenauer's policy of complete integration into the Western world. Eppler joined Heinemann's new party, the All-German People's Party (Gesamtdeutsche Volkspartei), in 1952, but like most members of the GVP, including Heinemann, he changed over to the Social Democratic Party of Germany (SPD) in 1956.