We examine the rise and mobilizational dynamics of social democracy, employing data reported by the Swedish authorities on the distribution of voting eligibility, turnout and partisan vote in local elections during the 1910s at a high level of disaggregation (by narrow income segments and administrative units). In line with the existing literature, we show that electoral socialism depended on both the extension of the suffrage to its ‘natural’ electorate, that is, the urban working class, and the organizational capacity of trade unions and other civic associations. In addition, we show that socialist support was not uniform within the working class – even for a highly homogeneous society. Instead, the social-democratic vote was initially stronger among low- to middle-income workers, only expanding to poor voters later in time. We complement our local data with an interwar panel analysis of socialist vote and post-war survey data.