"To what extent do interest groups adapt to a changing institutional environment? That is the central question of this article. Keating [2014. Introduction: rescaling interests, Territory, Politics, Governance 2(3)] distinguishes three drivers that can force interest groups to rescale: a functional, an identitarian and an institutional driver. The focus of this article is on the last driver of rescaling and on the assumption that interest groups increasingly operate on a regional scale when regional governments acquire more power. This ‘isomorphism hypothesis' is tested by analysing the adaptation of the principal labour and business groups in Belgium. We found that in Belgium, this hypothesis only accounts for a number of cases. The Belgian rescaling process is not only the result of institutional changes but also a response to economic differences between Flanders and Wallonia, as well as to the increasing salience of the language cleavage. Groups were rescaling even before the gradual federalization of the state started, but federalization has accentuated this process."

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