The literature on regionalist parties has traditionally focused on the origins of their electoral strength while their ideology remains an under-explored aspect of territorial party politics. This is surprising because for the question of whether decentralization ‘accommodates’ or ‘empowers’ regionalist pressure one needs to consider both. In this paper we single out the factors that increase the probability of adopting a radical (secessionist) as opposed to a moderate (autonomist) ideological stance, with a particular focus on the effect of decentralization. We make use of a large and original dataset, covering 11 countries, 49 regions, and 78 parties for the 1940s–2000s. Beyond the level of decentralization and decentralization reforms, we analyze the impact of two sets of factors: the first concerns regional identity and includes regional language, regional history and geographical remoteness; while the second concerns institutional/political variables which include voting systems, competition from statewide parties and from other regionalist parties, and office responsibility. We find that all variables matter for regionalist party ideology but with different effects across regional and national electoral arenas. We also find that level of decentralization and regional reform is significantly associated with radicalism, which suggests that policy success and accommodative strategies by statewide parties may lead to a polarization on the centre-periphery dimension.