The PSP also participated in these talks but broke off, because the majority of the PSP congress thought this alliance was neither pacifist nor socialist. The cooperative minority clashed strongly with the isolationist majority. In the 1971 elections the party lost two of its four seats, while the PvdA won seats. In 1972 the party's political leader, Hans Wiebenga (1917–2005) was replaced by the younger Bram van der Lek, who emphasized the environment as an important issue. He was unable to win seats in the 1972 elections. As party leader he would embrace extra-parliamentary protest of all kinds of groups: the PSP was involved in the nascent environmental, squatting, women's and students' movements. 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. From 1975 on, the party membership exhibited strong growth and doubled in the next five years. Nevertheless, the 1977 elections were disastrous: the party lost all but one seat – this is attributed to the political competition between the social-democratic prime minister Joop den Uyl and his Christian democratic competitor Dries van Agt, which caused many PSP-sympathizers to vote for Den Uyl. The internal dissent within the party also damaged its popular appeal. After one year Van der Lek left parliament, and he was replaced by Fred van der Spek. In the early 1980s the placement of American nuclear weapons became an important political issue. The PSP was involved in the organisation of national demonstrations against nuclear weapons and more than 80% of the members of the PSP attended one of the two mass protests against the placement nuclear weapons of 1981 and 1983. In the 1981 election the PSP was rewarded for its principled opposition: it won three seats. In the subsequent 1982 election it kept its seats. The party membership nearly reached 10,000 in this period. Since the 1980s the party began to cooperate more with the PPR, which had broken with the PvdA, and the CPN, which had destalinized. It cooperated mainly in municipal and provincial elections and legislatures, because a higher percentage of votes is necessary to gain seats in such elections.