In 1955 a group of "politically homeless" activists had formed. The group mainly consisted of former members of the Labour Party (PvdA) and the Communist Party of the Netherlands (CPN). They had left the PvdA over the military intervention against the Indonesian independence movement and the Labour party's support for NATO. Many of them had a background in the orthodox Marxist wing of the Social Democratic Workers' Party or the Christian Democratic Union (CDU), which had merged into the PvdA. The former members of the CPN had left their party over the Stalinist course of the CPN. There was also a group of these politically homeless that had never been members of parties, while others had been member of pre-war parties such as the Independent Socialist Party.