This article examines why Victorian Britain’s longest-running political controversy—the sixty-four-year campaign to legalise marriage between a widower and his deceased wife’s sister—was finally resolved in 1907. It explores why the Liberal government decided to support reform, what strategies it adopted and how it was able to force a deceased wife’s sister bill through an intransigent House of Lords that had, for more than half a century, succeeded in blocking similar bills. The article recognises that a multitude of factors—social, theological, imperial and legal—shaped opinion on the issue. Nonetheless, it argues that the Liberal government’s backing for marriage with a deceased wife’s sister (MDWS) should be viewed, in terms of its hegemonic public presentation, as the forgotten Liberal social reform. In keeping with the emerging New Liberalism, MDWS was packaged primarily as a social relief measure that spoke to the Liberal Party’s growing interest in the poor, child welfare, social justice, women’s rights and state support for workers. The commitment to easing the plight of those affected by the existing prohibition was real, but behind the public rhetoric lay a desire to stem nonconformist discontent among the Liberal Party’s electoral base, and an evolving constitutional crisis between the houses of parliament which gave the issue greater symbolic significance. For many peers, reform had become more than just palatable or desirable. It was necessary in the light of a change in colonial marriage law that had sought to address colonial grievances, promote imperial unity and better safeguard inherited wealth and social status.