17 pages, 6 figures.-- Submitted to Europ. Phys. J. B.We analyze a simple opinion formation model consisting of two parties, A and B, and a group I, of undecided agents. We assume that the supporters of parties A and B do not interact among them, but only interact through the group I, and that there is a nonzero probability of a spontaneous change of opinion (A->I, B->I). From the master equation, and via van Kampen's Omega-expansion approach, we have obtained the "macroscopic" evolution equation, as well as the Fokker-Planck equation governing the fluctuations around the deterministic behavior. Within the same approach, we have also obtained information about the typical relaxation behavior of small perturbations.We acknowledge financial support from Ministerio de Educación y Ciencia (Spain) through Grant No. BFM2003-07749-C05-03 (Spain). MSL and IGS are supported by a FPU and a FPI fellowships respectively (Spain). JRI acknowledges support from FAPERGS and CNPq, Brazil, and the kind hospitality of Instituto de Física de Cantabria and Departamento CITIMAC, Universidad de Cantabria, during the initial stages of this work. HSW thanks the European Commission for the award of a Marie Curie Chair at the Universidad de Cantabria, Spain.Peer reviewed