  The two dimensional electron gas (2DEG) in moderate magnetic fields in
ultra-clean AlAs-GaAs heterojunctions exhibits transport anomalies suggestive
of a compressible, anisotropic metallic state. Using scaling arguments and
Monte Carlo simulations, we develop an order parameter theory of an electron
nematic phase. The observed temperature dependence of the resistivity
anisotropy behaves like the orientational order parameter if the transition to
the nematic state occurs at a finite temperature, $T_c \sim 65 mK$, and is
slightly rounded by a small background microscopic anisotropy. We propose a
light scattering experiment to measure the critical susceptibility.
