A theoretical model is proposed for describing the polar biaxial and uniaxial nematic phases observed in thermotropic liquid crystals formed from rodlike polyester molecules. The polarity and biaxiality are shown to result from the same molecular mechanism, i.e., they are associated with the same critical order parameter which consists of two vectors determining the average molecular orientations in the biaxial and uniaxial phases. The model allows us to determine the critical behavior and electric or magnetic field effects that characterize polar nematic phases and to analyze, at the phenomenological and molecular levels, remarkable properties that have been disclosed experimentally.