  Recently we proposed a microscopic approach to the description of the phase
behaviour and critical phenomena in binary fluid mixtures. It was based on the
method of collective variables (CV) with a reference system. The approach
allowed us to obtain the functional of the Ginzburg-Landau-Wilson (GLW)
Hamiltonian expressed in terms of the CV (fluctuating densities). The
corresponding set of CV included the variable connected with the order
parameter. In this paper we use this approach to the study of the critical
behaviour of ionic fluids. For the restricted primitive model (RPM) we obtain
the functional of the grand partition function in the phase space of the two
fluctuating fields conjugate to the fluctuating densities. First we calculate
the phase diagram of the RPM in the mean-field (MF) approximation and then we
do this calculation taking into account the terms of the higher orders in the
effective Hamiltonian. In the both cases the phase diagrams demonstrate the
gas-liquid (GL) and charge ordering phase instabilities. In the latter case,
the obtained value for the GL critical temperature is in good agreement with
the MC simulation data whereas the critical density is underestimated. The
explicit expression found for the grand thermodynamic potential in the vicinity
of the GL critical point implies a classical critical behaviour of the RPM.
