In January 1937 the Socialist League, the Communist Party and the ILP together issued a ‘Unity Manifesto’, expressing a number of common aspirations. The three groups then embarked on a series of mass meetings and demonstrations which were vigorously denounced by Transport House and which delighted the Press, always eager to make the most of a Party quarrel. On paper, the joint activity was directed ‘against Fascism, reaction and war and against the National Government’. In practice it was closely linked to a vigorous campaign to secure Communist affiliation to the Labour Party.

In Michael Foot's view, the Unity Campaign was ‘the most ambitious bid made by the British Left throughout the whole period of the thirties to break the stultifying rigidity of Party alignments’. Yet the aim of ‘unity’ – united action by the Labour Party and extreme left groups outside it – was a lost cause from the start; and a tangled skein of irreconcilable views on doctrine and tactics produced action by the Campaign's participants which was barely linked to any consistent purpose. The main effects of the campaign were to weaken the Labour Left, destroy the Socialist League, deepen existing divisions within the Labour Party, and queer the pitch for later attempts to bring opposition forces together in order to bring about a change in Government policy.