This study develops a conceptualisation of the political cultures of the Chilean Socialist Party (PSCh) in order to understand and develop an explanation of the nomination of Michelle Bachelet as presidential candidate in 2005 which avoids the oversimplifications of existing approaches. At the theoretical level, political culture is defined from the collective action perspective conceptualising it as a framework for action (Elkin, 1993: 123). This political culture is formed by patterns for political participation which drive internal power relations between members and leaders. The formation of these patterns is influenced by their social context which is mediated by groups of members and leaders in relation to their histories, experiences of critical historical junctures and ideological heritages. The conceptualisation developed thus links structure and agency in a way that enables a nuanced analysis of inter-party power relationships and agency. This fosters a deeper explanation of Bachelet’s nomination and enables evaluation of its meaning for the party as an institution. It also help us to understand internal dynamics and contradictions than could be found in her nomination. I argue that the PSCh has historically had two political cultures whose relative balance of power within the party has shifted over time. The first which I name the institutional pattern is a normative pattern which frames political participation as libertarian, democratic and pluralistic. Currently it is the non-dominant political culture to be found in the party bases. The second is the dominant party political culture which I name the practice pattern. This frames relationships between members and leaders from the perspective of co-optation, authoritarianism and hegemony over decision-making. The formation of these political cultures has been influenced by the Chilean social context during three stages of party institutional development: Foundational (1933-1956), the New Left (1956-1979), and Socialist Renewal (1979-2005). The first political culture represented in the institutional pattern is formed during the Foundational stage, where political participation is based on Pizzorno’s system of solidarity and collective identification. The second political culture, represented by the practice pattern integrated ideas about discipline and obedience presented in the New Left stage but is reinforced during the Socialist Renewal stage, when participation shifted to Pizzorno’s system of interest and individual goals. These two political cultures define and delimit participation within the party, which is highly individualised. Membership and leadership participation is settled between factions, but also results in the presence of informal types of membership and leadership within the party. Factional membership and leadership is the attribute which legitimises a subject as party member. In Bachelet’s case, her persona brings together these two cultures, despite increasing tensions between excluded sectors in the base and leadership of the party and the hegemonic leadership. The first institutional pattern played to an idea that her candidacy and nomination represented the inclusion of historical members as part of pluralism and democracy. The discontented base membership linked her persona with this pattern and supported her. However, as a faction leader and mandatario she also reinforced the elitist and hegemonic 'practice pattern' of participation, which resulted in strong disciplinary relationships coming from the faction’s elite in order to secure her nomination. As a result, the elitist practice pattern was deepened due to the strengthening of authoritarianism, co-optation and hegemony within the party. This then helps us understand the fragility of party unity in support of her candidacy and the subsequent division of the party in 2010 when a coalition of the right was elected to power.