The use of social media by parliamentarians is opening up a new communication arena. In Catalonia, where 85 percent of parliamentarians have a Twitter account, two questions emerge from this new social phenomenon. First, who are the opinion leaders of the parliamentarians' online political networks, and second, do the characteristics of the Members of Parliament' (MPs') Twitter networks and the attributes of the parliamentarians influence the likelihood of forming communication ties? This article seeks to ascertain whether social media are challenging party politics and leadership in communication flows in Parliament. We used a social network analysis of relationships among the Catalan parliamentarians with Twitter accounts (115 of the 135 members) to reveal the potential influencers of the following-follower network. Exponential Random Graph models were employed to determine the endogenous (network) and exogenous (node attributes) factors facilitating MPs' communication ties. We found evidence that Catalan MPs' communication ties arise from network dynamics (reciprocity and popularity) and from MPs' political position. We also discovered that new potential influencers, who are not official party leaders and do not play important roles in Parliament, are emerging as brokers within the Catalan parliamentary Twitter network.