The core support for social democratic parties has long rested in labour union members. That is no less true for the Swedish Social Democratic party (SAP) in relation to the main industrial labour union (LO). The support for SAP among LO-member has however faltered in recent years, and increasingly the core voters are lending their support to the Sweden Democrats (SD), a right-wing populist party. The purpose of this study was to identify possible explanations as to why the core voters have abandoned the party, with an emphasis on the party’s own actions and strategy. The method to achieve this were extensive interviews with experts at LO, through which both pre-existing as well as new modells of explanations were developed and presented. Crucially the factors and models intended to explain this occurrence is thus rooted in an union perspective, since the interviewees are expected to express the clearest analysis of the situation, as well as represent the interests of their members. The study has found a number of possible reasons for the core voter flight. 1) Underlying reasons, i.e changes in the socioeconomic structure, due to among other things urbanisation, increased employment and education opportunities – which has eroded working class consciousness and thus the degree of class voting. Furthermore SAP has exacerbated the situation, either 2) subconsciously by adopting new frames of references and changing the party’s class character, or 3) consciously by pursuing a supra-class strategy in order to maximize votes. Regardless of how or why the party currently lacks its former ability to appeal to its working class base. Furthermore it can in part be ascribed to a prioritizing of strategic aims above ideological ones. However, the party can reverse the trend of core voter flight by refocusing on classic social democratic policies of redistribution, welfare, and class politics.