Defence date: 14 December 2018 ; Examining Board: Prof. Stefano Bartolini, European University Institute; Prof. Mark Bovens, Utrecht University; Prof. Daniel Gaxie, University Paris 1 Panthéon-Sorbonne (Co-supervisor); Prof. Hanspeter Kriesi, European University Institute (Supervisor) ; The aim of this dissertation is to shed new light on the electoral support for radical rightwing parties (RRP). Whereas most existing investigations assume a form of causal homogeneity, the starting point of this research project is based on what I call electoral equifinality : the coexistence of multiple causal paths leading towards different forms of support for the same political party. In order to discern and understand different forms of RRP support, the study takes both the supply and the demand side into account. Regarding the supply side, I link cleavage theory and conflict sociology to the Laclauian notion of equivalence , arguing that the electoral appeal of RRP relies on their capacity to coherently unify a multiplicity of heterogeneous demands along the same main antagonism: national versus foreign. Following Weber’s and Parkin’s thoughts on social closure, I theorize that this nativist core conflict is invoked according to a specific tripartite structure, which, to my knowledge, has remained quite unnoticed in the existing literature. In accordance with this theory, a new dataset is developed (n = 1,378), based on the tweets of Le Pen and Wilders, to compare the political supply of their parties in terms of forms of closure, reference groups and issue categories. Pertaining to the demand side, a sequential mixed methods design is followed, focusing quantitatively on the structural heterogeneity within RRP constituencies along three dimensions within a Bourdieusian framework of social space: social characteristics (who); political preferences (why) and political interest (how). Subsequently, the second qualitative research step is based on life history interviews with 125 RRP voters in France and the Netherlands, ...