  Mercury is the unique known planet that is situated in a 3:2 spin-orbit
resonance nowadays. Observations and models converge to the same conclusion:
the planet is presently deeply trapped in the resonance and situated at the
Cassini state $1$, or very close to it. We investigate the complete non-linear
stability of this equilibrium, with respect to several physical parameters, in
the framework of Birkhoff normal form and Nekhoroshev stability theory. We use
the same approach adopted for the 1:1 spin-orbit case with a peculiar attention
to the role of Mercury's non negligible eccentricity. The selected parameters
are the polar moment of inertia, the Mercury's inclination and eccentricity and
the precession rates of the perihelion and node. Our study produces a bound to
both the latitudinal and longitudinal librations (of 0.1 radians) for a long
but finite time (greatly exceeding the age of the solar system). This is the
so-called effective stability time. Our conclusion is that Mercury, placed
inside the 3:2 spin-orbit resonance, occupies a very stable position in the
space of these physical parameters, but not the most stable possible one.
