We consider the effect of primary elections on party membership and electoral behaviour. Direct democracy instruments trigger significant changes in the role and behaviour of grassroots members. The case of the Italian centre - left parties, and particularly the Democratic Party, is in this sense relevant, as for over a decade these parties have been reaching out to supporters in order to include them into decision-making processes, such as the selection of party leaders and candidates to legislative and executive offices. The distinction between members and supporters has blurred. The article focuses on voting behaviour and party attachment of three different groups of primary voters – namely, party members, supporters, and external voters. What is the difference between these three groups with regard to voting behaviour and motivations in primary elections? And what is the difference with regard to voting intentions in general elections? We examine these issues using original survey data collected in 2012 during the centre - left coalition’s primary elections. We highlight the consequences of the differences between members and supporters with regard to their voting behaviour and motivations.