  A Landau-Ginzburg functional of two order parameters (charge-density $\phi$
and mass-density deviation $\eta$) is developed in order to yield a field
theory relevant to ionic lattice gases as well as a family of off-lattice
models of ionic fluids that go beyond the restricted primitive model (RPM). In
a mean-field (MF) approximation an instability of a uniform phase with respect
to charge fluctuations with a wave-number $k\ne 0$ is found. This second-order
transition to a charge-ordered phase terminates at a tricritical point (tcp).
Beyond MF, a singularity of a mass correlation function for $k\to 0$ occurs at
ion concentration lower than that of the MF tcp. An effective functional
depending only on $\eta$ is constructed. For low ion concentration the usual
Landau form of the simple-fluid (Ising) functional is obtained; hence in this
theory the critical point is in the Ising universality class.

Statistical Mechanics;Soft Condensed Matter