The presence of multiple polarized beams can limit the polarimetric discrimination capability of a wire-grid polarizer (WGP). In this study, the effect of linearly polarized background on the polarimetric performance of a WGP has been investigated and compared with that of a perfect polarizer. Simulation results based on rigorous coupled-wave analysis indicate that while a WGP mimics a perfect polarizer in discrimination characteristics, the range of the object polarization angle that can be discriminated against polarized background is fairly limited. The negative impact of Rayleigh anomaly is also discussed. The detectability of the object polarization can be strongly enhanced by employing a multi-cell WGP with multiple polarization orientations.