Party researchers often claim that party leaderships wield disproportionate power within their parties. We argue that earlier research has undervalued a bottom-up mass party perspective. We claim that party leaderships must heed internal party arenas in order to hold their parties together. In order to explain why the Swedish Social Democrats – a key case considered by previous research to be dominated by its leaders – have changed ideology 1968-2013, we add an explanatory factor concerning ideological shifts among party conference delegates to the more established factors concerning election loss, party system change and party leadership change. The Left-Right positions of Social Democratic party conference delegates were mapped through an expert survey. Party ideology positions were mapped through data Comparative Manifesto Project data. The results show that there is equally strong support for a distinctly more bottom-up perspective as for the established elite perspective.