ABSTRACT This article argues that a ‘constitutional lens’ sheds light on British political contestation in the 1990s. It is commonly suggested that 1990s politicians merely consolidated the hegemony of ‘Thatcherism’. Yet, this struggles to encompass the decade’s political volatility, from the Conservatives’ descent into internecine warfare over the Maastricht Treaty to the fraught Irish peace process. Though varied, these developments shared a key theme: controversy over the constitution of the United Kingdom. To illustrate its case, this article applies the constitutional lens to the 1990s left. Existing accounts emphasise Labour’s concessions to Conservative opponents. By 2001, however, the first Blair government had enacted contentious constitutional changes: devolution, proportional voting systems, and a Human Rights Act. These policies reflected a dynamic, new, liberal-left politics of constitutional reform. From the 1980s, liberals, social democrats and Marxists forged a new constitutional agenda, responding to trends since the 1960s—growing ‘popular individualism’, agitation for ‘human rights’, European integration, and Scottish nationalism—and to the end of the Cold War. By the 1990s, campaigners had developed a bold programme for ‘modernising the constitution’, which influenced Labour’s programme. Through New Labour’s governments, this agenda profoundly reshaped the institutions of British politics in the twenty-first century.