  We present photometric and spectroscopic observations of SN 2007if, an
overluminous (M_V = -20.4), red (B-V = 0.16 at B-band maximum), slow-rising
(t_rise = 24 days) type Ia supernova in a very faint (M_g = -14.10) host
galaxy. A spectrum at 5 days past B-band maximum light is a direct match to the
super-Chandrasekhar-mass candidate SN Ia 2003fg, showing Si II and C II at
~9000 km/s. A high signal-to-noise co-addition of the SN spectral time series
reveals no Na I D absorption, suggesting negligible reddening in the host
galaxy, and the late-time color evolution has the same slope as the Lira
relation for normal SNe Ia. The ejecta appear to be well mixed, with no strong
maximum in I-band and a diversity of iron-peak lines appearing in
near-maximum-light spectra. SN2007 if also displays a plateau in the Si II
velocity extending as late as +10 days, which we interpret as evidence for an
overdense shell in the SN ejecta. We calculate the bolometric light curve of
the SN and use it and the \ion{Si}{2} velocity evolution to constrain the mass
of the shell and the underlying SN ejecta, and demonstrate that SN2007 if is
strongly inconsistent with a Chandrasekhar-mass scenario. Within the context of
a "tamped detonation" model appropriate for double-degenerate mergers, and
assuming no host extinction, we estimate the total mass of the system to be 2.4
+/- 0.2 solar masses, with 1.6 +/- 0.1 solar masses of nickel-56 and with
0.3-0.5 solar masses in the form of an envelope of unburned carbon/oxygen. Our
modeling demonstrates that the kinematics of shell entrainment provide a more
efficient mechanism than incomplete nuclear burning for producing the low
velocities typical of super-Chandrasekhar-mass SNeIa.
