  We study, by using liquid-state theories and Monte Carlo simulation, the
behavior of systems of classical particles interacting through a finite pair
repulsion supplemented with a longer range attraction. Any such potential can
be driven Ruelle-unstable by increasing the attraction at the expense of
repulsion, until the thermodynamic limit is lost. By examining several
potential forms, we find that all systems exhibit a qualitatively similar
behavior in the fluid phase as the threshold of thermodynamic stability is
approached (and possibly surpassed). The general feature underlying the
approach to Ruelle instability is a pronounced widening of the liquid-vapor
binodal (and spinodal) line at low temperatures, to such an extent that at the
stability threshold a vanishing-density vapor would coexist with a
diverging-density liquid. We attempt to rationalize the universal pathway to
Ruelle instability in soft-core fluids by appealing to a heuristic argument.
