The aim of this paper is to propose and to estimate an explanatory model for the determinants in the growth of public spending in Spain during the last three decades, in this way providing an approach to one of the main changes that has been experienced by the Spanish public finance in the last two centuries. The model which is proposed falls within the socalled neoclassical political economy, in such a way that the size of the public sector and, as a consequence, the volume of spending, will be the result of a political process in which the interests of all the agents or groups that participate in that process interact. The estimation of the model, by way of the cointegration technique, provides an explanation for the evolution of public spending as being a conjunction of short and long term elements, where the demand factors, driven forward by changes in social and economic organisation and the supply factors, linked to the appearance of a new market of political interchange following democratisation, are intertwined, with both supply and demand being accentuated by the economic crisis and by the thorough reform to the tax structure which resulted in the Treasury being given the resources necessary to increase the size of the public sector.