 In 1981, he was among the Labour MPs who defected to the new Social Democratic Party.Sandelson later said that he had decided to join the party months before, and had voted for Michael Foot in the Labour leadership election in order to ensure Labour had an unelectable leader.In 1983 he lost his seat, polling 29% of the vote - although he almost pushed Labour into third place which allowed the Conservative candidate Terry Dicks to win.