Nellist was a prominent figure in organising the Socialist Alliance, locally and across the UK, as a loose formation of individuals and groups. He became the Chair of the Socialist Alliance, but resigned in 2001, in protest of what the Socialist Party saw as manoeuvrings of the Socialist Workers Party to take control of the Alliance. He is involved in the Campaign for a New Workers' Party in Britain, which is a Socialist Party sponsored campaign to create a new party to represent the working class in the UK.