In 1902, Middleton moved to work on the "Harringay Mercury", and became the first Assistant Secretary of the Labour Party. He remained in this role for many years, a close supporter of Ramsay MacDonald. He opposed World War I, founding the War Emergency Workers' National Committee, and was initially enthusiastic about the October Revolution. In 1920 he was one of the joint secretaries of the National Council of Action alongside Fred Bramley and H. S. Lindsay.