Pulsed electric fields are used to study the influence of a strong field on the nematic-isotropic phase transition for cyanobiphenyl and stilbene-type liquid crystals. Deviation of the electric field-induced optical anisotropy from the Kerr law is observed and it is shown that such electric fields can shift the transition temperature substantially. The induced birefringence and the shift of the transition temperature are measured as a function of the electric field strength. The results are explained qualitatively in the context of the Landau-De Gennes theory with two order parameters. The coefficients of the phenomenological theory are calculated using the simple density functional theory of polar nematics developed in this paper and the results for the shift of the transition temperature are compared with experiment results.