The 2011 British referendum on the electoral system offered voters a change within the majoritarian family from single-member plurality to the alternative vote. The alternative vote is not proportional, but the ‘yes’ campaign in the United Kingdom included small parties and ‘democracy sector’ organisations previously associated with advocating proportional representation. This anomalous behaviour can be explained by applying social movement theory, especially interpretations related to political opportunity structures and frame analysis. The Liberal Democrats, Electoral Reform Society and others had previously criticised the alternative vote, yet decided to campaign in favour of it. This led to an unclear framing of their objectives and, ultimately, their failure.