This article examines the relationship between the British Labour Party and its affiliated trade unions under the Blair government. It shows that the party–union link in Britain, contrary to expectations, was not characterized by further institutional de‐linkage under Blair. The article explains why, using a political exchange model focusing on the unions’ supply of votes and finance in return for policy concessions. It is shown that the capacity to enforce deals is paramount in political exchange, but that the unions’ two major enforcement mechanisms – intra‐party and industrial power – were both diminished. In the absence of enforceable deals, exchange models predict the undersupply of resources, which happened as the unions withdrew funds. Once Labour’s continued reliance on union money became evident, the party offered a comprehensive policy bargain – the Warwick agreement – to guarantee union donations. The agreement, with its institutional innovations, entailed the partial re‐institutionalization of political exchange (‘re‐linkage’), with the possibility of further funding cuts if the party reneged on the deal. At present, only comprehensive state funding for parties could threaten the link.