During the 1920s, Harold Laski worked on producing a comprehensive account of a political philosophy appropriate for the new age of social democracy that was just emerging. During the 1930s, however, his optimistic political outlook waned and with this he modified his position and, in its place, presented a materialist account of British constitutional arrangements. This paper explains this later development of his thought. It examines the unfolding of his argument through his studies of the crisis of parliamentary democracy, the nature of the modern state and his materialist analysis of the British constitution and offers critical reflections of the significance of this phase of his work.