The steady-state electrorheological (ER) properties of unidomain nematic poly(n-hexyl isocyanate) (PHIC) solutions inp-xylene have been experimentally characterized and theoretically modeled with a two-dimensional modified version of Doi's theory of liquid crystalline polymer rheology. This molecular model allows the prediction of ER fluid behavior directly from properties of the component polymer and solvent. Optical conoscopic measurements, confirmed that high applied electric fields align the PHIC molecules in the field direction, so the high-field viscosity approaches the transverse Miesowicz viscosity, eta(c) The PHIC solutions exhibited many desired ER properties comparable to strong suspension ER fluids, such as a transmitted shear stress of 16 kPa and a viscosity enhancement of 180 times. At low shear rates, quantitative agreement between predictions and data at high fields was obtained using a single adjustable parameter, the rotational diffusivity, D-r. At high shear rates, an additional parameter, beta, was required to account for the viscous contributions to the stress tensor. The temperature dependence of the solution electrorheology was predicted to come exclusively from that of the solvent viscosity.