The data consists of transcripts of elite interviews conducted by team members in the period October 2019-July 2021.The parties selected for investigation are all seasoned populist radical right parties which are well established within their party systems: the Lega per Salvini Premier (League – League for Salvini Premier); the Vlaams Belang (VB – Flemish Interest); Perussuomalaiset (PS – the Finns Party); and the Schweizerische Volkspartei/Union démocratique du centre (SVP/UDC – Swiss People’s Party). The selection offers variation in terms of the parties' national setting (federal vs. unitary systems; EU vs. non-EU countries), foundational characteristics (ex-nihilo organisations, such as the VB, vs. converted parties, such as the League, PS and SVP), and executive experience (all parties except VB). At the intra-party level, our sample covers three areas for each party. We have compared locations where support has traditionally been strong (e.g. Lombardy, Antwerp, Satakunta, Zurich) with others in which the parties have grown more recently and/or with more difficulty (e.g. Emilia-Romagna, East Flanders, Central Finland, Geneva). Therefore, in our selection of areas at the sub-national level we have sought to include both typical and less typical cases, allowing us to reveal diverging cultures and narratives within each party. This has helped us to provide a holistic picture of the varied ideological and organisational features of these parties at the local level, allowing detailed comparisons to be made. The research team conducted 125 original semi-structured elite interviews (EIs) with (leading) national and local party representatives and executives. Of these 93 are included here (i.e. interviews with respondents who have agreed that their transcript can be deposited with Reshare). The interviews were conducted with representatives from each of the four parties, with a well balanced mix of national representatives (MPs, national leaders, etc.), regional representatives (e.g. regional councillors), and local representatives (council representatives, Mayors etc.). Interviews were mainly used to identify and explain the reasons behind the fostering of the mass party organisational model by populist radical right parties, which can shape our understanding of party organisational development more generally. We deployed a thematically common interview schedule through which interviewers elicited accounts of organisational practice. Although based on an original set of questions we had devised ourselves, our exploration was inspired by previous scholarly work, specifically Susan Scarrow’s (2015) conceptualisation of the roles members fulfil inside and outside party organisations, and Richard Katz’s (2005) list of incentives parties have to retain a large membership. From the evidence collected we identified the party-building strategies of the party elites and the reasons for investing in and socialising the grassroots. We also interpreted similarities and differences between the parties under investigation.It is often claimed that political parties in Europe are losing their traditional function of bridging the gap between citizens and the political elites. Whereas the 20th century saw the rise of political parties characterised by large memberships organised in local branches, it is now widely assumed that the era of the 'mass parties' is over. In a well-known article, for instance, Katz and Mair (1995) argued that political parties are converging towards a new organisational model: the 'cartel party'. In essence, this model entails that parties become increasingly intertwined with the state, whilst ties with their grass-root membership, or what is left of it, weaken. Parties, in other words, become actors of the state rather than society. We argue, however, that it may be too soon to speak of the end of the mass party. Particularly among certain parties challenging the traditional political establishment (or 'cartel'), this party model is ostensibly still popular. In our study we focus in particular on parties of the populist radical right (PRR), which currently pose the most serious electoral threat to mainstream parties and which, despite several prominent exceptions, have also been shown to often adopt mass organisations and create communities of loyal partisans activists (Heinisch and Mazzoleni, 2016). PRR parties, then, do not only pose a challenge to established politics, but also to the theory that the age of mass parties is over. In our research we aim to understand why the mass party is still a popular model among the PRR, from the perspective of both the party members and party elites. On the one hand, we seek to understand why different groups of people become activists of these parties and what different typologies of party activists contribute to their parties. On the other, we study why the leaderships of populist parties go against the 'tide of disengagement' characterising their competitors: what are, in their eyes, the advantages of adopting rooted models of party organisation. Crucially, answering these questions has allowed us to build a more nuanced conceptual framework for assessing differential party development. We take a comparative case study approach, and investigate the 'life of the party' within four seasoned PRR parties which are well established within their party systems: the Italian League, the Flemish Interest in Belgium, The Finns Party in Finland, and the Swiss People's Party. To map the parties' formal and informal organisational structures, and shed light on party members' and elites' motivations, our study combines a variety of methods: it involves a study of secondary and party literature, an original survey conducted among party members, life-history interviews with party activists, and semi-structured elite interviews with party representatives and executives. By advancing our knowledge of what happens inside PRRPs, we seek to address the serious shortage of comparative party research on this topic. The project will also have benefits beyond academia, as we will listen to the activists of populist parties themselves, and capture in detail the individual, cultural, social and organisational drivers of populist sentiment at a highly critical juncture in the life of the EU. Our findings thus have implications not only for political parties more generally, but also for organisations interested in the quality of Western democracy.

Elite interviews with political representatives at national, regional and local levels from the following parties: the League (Italy); The Swiss People's Party; the Finnish Interest (Belgium); the Finns Party. Snowball sampling procedures.