Whilst some authors have classified nationalist and regionalist parties as ‘niche’ parties, this contribution argues that such a categorization is of limited usefulness for understanding the ideological nature of this party family. The ideological evolution of two parties—Plaid Cymru in Wales and the Galician Bloque Nacionalista Galego (BNG)—is mapped in order to demonstrate how their core ideology (the territorial re-configuration of political authority) has been moderated and added to over time. As a result of this ideological adaptation, it is argued that these parties can no longer be described accurately as ‘niche’ parties. Ideological adaptation is attributed to the pressures associated with the pursuit of vote-seeking, policy-seeking and office-seeking goals. Such pressures come to bear on any political party that seeks to evolve from being a party of protest to being a party in power, and nationalist and regionalist parties are not immune to the dilemmas and risks associated with a change in the status of a political party (for example, as a result of passing the thresholds of representation and/or government). However, it is also necessary to give due consideration to the multi-level political context which is intrinsic to the political and electoral strategies of regionalist and nationalist parties, if the causes and implications of these actors' ideological evolution are to be fully understood.