  We present photometry, spectra, and spectropolarimetry of supernova (SN)
2012ab, mostly obtained over the course of $\sim 300$ days after discovery. SN
2012ab was a Type IIn (SN IIn) event discovered near the nucleus of spiral
galaxy 2MASXJ12224762+0536247. While its light curve resembles that of SN
1998S, its spectral evolution does not. We see indications of CSM interaction
in the strong intermediate-width emission features, the high luminosity (peak
at absolute magnitude $M=-19.5$), and the lack of broad absorption features in
the spectrum. The H$\alpha$ emission undergoes a peculiar transition. At early
times it shows a broad blue emission wing out to $-14{,}000$ km
$\mathrm{s^{-1}}$ and a truncated red wing. Then at late times ($>$
100$\,$days) it shows a truncated blue wing and a very broad red emission wing
out to roughly $+20{,}000$ km $\mathrm{s^{-1}}$. This late-time broad red wing
probably arises in the reverse shock. Spectra also show an asymmetric
intermediate-width H$\alpha$ component with stronger emission on the red side
at late times. The evolution of the asymmetric profiles requires a density
structure in the distant CSM that is highly aspherical. Our spectropolarimetric
data also suggest asphericity with a strong continuum polarization of $\sim
1-3$% and depolarization in the H$\alpha$ line, indicating asphericity in the
CSM at a level comparable to that in other SNe IIn. We estimate a mass-loss
rate of $\dot{M} = 0.050\, {\rm M}_{\odot}\,\mathrm{yr^{-1}}$ for $v_{\rm pre}
= 100$$\,$km$\,$$\mathrm{s^{-1}}$ extending back at least 75$\,$yr prior to the
SN. The strong departure from axisymmetry in the CSM of SN 2012ab may suggest
that the progenitor was an eccentric binary system undergoing eruptive mass
loss.
