  A conclusive model for the formation of dwarf spheroidal (dSph) galaxies
still remains elusive. Owing to their proximity to the massive spirals Milky
Way (MW) and M31, various environmental processes have been invoked to explain
their origin. In this context, the tidal stirring model postulates that
interactions with MW-sized hosts can transform rotationally supported dwarfs,
resembling present-day dwarf irregular (dIrr) galaxies, into systems with the
kinematic and structural properties of dSphs. Using N-body+SPH simulations, we
investigate the dependence of this transformation mechanism on the gas
fraction, fgas, in the disk of the progenitor dwarf. Our numerical experiments
incorporate for the first time the combined effects of radiative cooling,
ram-pressure stripping, star formation, supernova (SN) winds, and a cosmic UV
background. For a given orbit inside the primary galaxy, rotationally supported
dwarfs with gas fractions akin to those of observed dIrrs (fgas >= 0.5),
demonstrate a substantially enhanced likelihood and efficiency of
transformation into dSphs relative to their collisionless (fgas = 0)
counterparts. We argue that the combination of ram-pressure stripping and SN
winds causes the gas-rich dwarfs to respond more impulsively to tides,
augmenting their transformation. When fgas >= 0.5, disky dwarfs on previously
unfavorable low-eccentricity or large-pericenter orbits are still able to
transform. On the widest orbits, the transformation is incomplete; the dwarfs
retain significant rotational support, a relatively flat shape, and some gas,
naturally resembling transition-type systems. We conclude that tidal stirring
constitutes a prevalent evolutionary mechanism for shaping the structure of
dwarf galaxies within the currently favored CDM cosmological paradigm.
