The paper focuses on a neglected group of territorial politics: the dominant non-territorially defined political parties, mainly catch-all parties of Christian Democratic or Social Democratic origin. In contrast to regionalist parties, they do not have incentives for territorial reforms, yet they are engaged in them. Despite a state of mutual neglect between the literature on territorial politics and the catch-all party branch of party theory, we argue that the catch-all party model does help understand territorial party politics in two steps. Territorial reforms as practical tests put catch-all parties under tremendous stress, since particularistic regionalist interests may threaten the catch-all party formula of competitiveness with its symmetry of interest group integration. According to the degree of the threat, catch-all parties take countermeasures in territorial reforms, restoring the respective conditions again. To test this hypothesis and identify this mechanism of ‘threat and response’, we scan territorial reforms in Germany, Austria, Great Britain and Italy.