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.