We propose an extension of the Landau theory of phase transitions to apply to potassium-dihydrogen-phosphate-type ferroelectrics. The polarization at point x can interact with the polarization at a distant point y through a temperature-dependent kernel G(x,y,T). The essence of the proposal is the assumed form for G. The form chosen is such that the dipolar interaction can be described by a free energy with a long-range kernel decreasing exponentially. We also study the relation between the various possible critical exponents and the determination of the variables that describe the model. Finally, of special interest is the problem of crossover from fluctuating polarized clusters to static domains as temperature decreases below ${\mathit{T}}_{\mathit{c}}$.