  Based on the non-skeleton diagrammatic expansion satisfying the
compressibility and spin-susceptibility sum rules, we investigate static charge
and spin responses in a two-dimensional extended Hubbard model with the
nearest-neighbor Coulomb repulsion in the vicinity of its charge-ordering
transition point. In this expansion, we can calculate approximate charge and
spin response functions by systematic inclusion of vertex corrections, from
which we obtain the uniform susceptibility equal to the so-called q-limit of
the response function and the second-order transition point as a divergent
point in the same response function at some finite wave-number vector. It is
shown that the reentrant charge-ordering transition, which has already been
observed in the random-phase approximation (RPA), remains to take place even
though the vertex corrections are included beyond the RPA. As a prominent
effect of the vertex corrections, we find that the uniform spin susceptibility
is enhanced due to charge fluctuations developing toward the charge-ordering
transition. We give a qualitative comparison of this enhanced spin
susceptibility with the experimental results on the quasi-two-dimensional
organic conductors, together with its explanation in the Landau's Fermi-liquid
theory.
