The Coulombic attraction theory of colloid stability, proposed by Sogami and Ise, is adapted to the properties of the two-phase (gel) region of colloid stability which is observed in clay swelling. A review of the recent debate about the nature of the thermodynamic electrostatic interaction potential between colloidal particles in electrolyte solutions is given, and the essential experimental facts about the n-butylammonium vermiculite system are recalled. In the two-phase region, it is shown that Sogami theory combined with the Dirichlet boundary condition (constant surface potential) yields the prediction that the ratio (s) of the salt concentration in the supernatant fluid to the average salt concentration in the gel phase be constant. For a surface potential of 70 mV, s is equal to 2.8, in excellent agreement with the experimental results on the n-butylammonium vermiculite gels. This result is used to calculate the interlayer spacings in the gel phase as a function of the electrolyte concentration (c) and initial volume fraction (r) of the clay in the solvent medium, again in excellent agreement with recent studies of n-butylammonium vermiculite swelling. This in turn yields a prediction for the position of the r-c phase boundary between the one-phase and two-phase regions of colloid stability, which is also expressed in terms of the position of the minimum in the pair potential between parallel clay plates. This is a central prediction of the Sogami theory, in which there is a weak attractive tail in the electrical interaction potential. The standard theory of colloid stability, the DLVO theory, is shown to be a limiting case of the Coulombic attraction theory, in the one-phase region.