Both the moderates and the most radicals left the party. Until 1974 a Trotskyist group, the Proletarian Left, led by Erik Meijer, now MEP for the Socialist Party operated within the party because they wanted to use the PSP to electrify the masses. In 1974 nearly all of them (except for their leader Meijer) left the party to found what later became the group Socialist Alternative Politics. In 1975 the moderate so called "progressive cooperatives" left the party. Many members of them joined the PvdA.