Why institutionalisation comes about? From a social movement perspective grounded on the political process approach, this paper proposes an analytical framework for understanding the shift towards more routinised tactics to the detriment of extra-conventional strategies during the declining phase of the Spanish cycle of protest against austerity and for real democracy. It focuses on one specific instance of institutionalisation, the emergence of the new challenger party Podemos. I contend that institutionalisation, understood as the strategic and deliberate shift from extraconvtentional towards more institutional forms of action, comes from a concatenation of three mechanisms: 1) appropriation of political opportunities via electoral dealignment, 2) construction of symbolic leadership, 3) cognitive liberation. Given the absence of facilitation, persisting grievances and exhaustion of extra-conventional repertoires of action, the liberation of some cognitive cues allows some activists to reconsider–– and eventually change–– the movement strategy as a way to redress those grievances more effectively through coordinated action. Therefore, the same three sets of mechanisms that the political process tradition has identified at the core of the mobilisation process may also be helpful to explain institutionalisation processes that take place during the demobilisation phase of the cycle. Information from twelve semistructured interviews conducted with key informants and descriptive statistics that come from a number of different sources are used to illustrate the core argument.