Internet offers opportunities for political mobilization that parties are underexploiting. If we believe that parties have as their main goal maximizing support the question that follows is, why are parties not using the Internet to further this goal? I propose a model that brings light into this paradox and accounts for observed differences in party behavior online. I argue that if parties are not using the new media to mobilize support is because the benefits of using it are not clear and there are costs -- communicational and organizational -- involved in this decision. Parties that are large and in the opposition, that are either low or highly cohesive ideologically, and that have small extra-parliamentary organizations will have the greatest incentives to use the Internet for political mobilization and will be in the best position to pay the costs involved in this decision. The paper uses evidence from parties in Spain and Catalonia to illustrate the argument.