In this paper, we explore the mechanisms that triggered the rise of social democracy, the collapse of liberalism, and, as a result, the overall transformation of the Left. We claim that social democracy was electorally successful when two conditions obtained: the franchise had been extended to a significant fraction of the working class; and socialist candidates could count on the organizational support of labor unions. In turn, we stress the importance of the strategic and institutional choices made by non-socialist (mainly, liberal) politicians: whenever they preferred to cater to the interests of the middle classes over the demands of urban workers and/or could not find a way to construct a supra-class electoral strategy including all non-conservative voters, unions backed, sponsored or promoted socialist parties and candidates. We present an empirical analysis of the rise of the Labour party in England and Wales between 1900 and 1935 based on a novel, longitudinal constituency-level dataset that includes information on electoral returns, class composition of the electorate and the union density. We provide evidence of the class basis of the electoral success of the Labour party, but also of the important role played by the unions.