  In this paper we investigate the role of the electronic correlation on the
hole doping dependence of electron-phonon and superconducting properties of
cuprates. We introduce a simple analytical expression for the one-particle
Green's function in the presence of electronic correlation and we evaluate the
reduction of the screening properties as the electronic correlation increases
by approaching half-filling. The poor screening properties play an important
role within the context of the nonadiabatic theory of superconductivity. We
show that a consistent inclusion of the reduced screening properties in the
nonadiabatic theory can account in a natural way for the $T_c$-$\delta$ phase
diagram of cuprates. Experimental evidences are also discussed.
