A microscopic statistical theory of diblock copolymer melts in an electric field is developed within the framework of the random phase approximation. We predict that near the spinodal point copolymer melts reveal four different universal types of behavior. In determining to which universal class the copolymer melt belongs, the only relevant parameters turn out to be f, the fraction of monomers A in a chain, and parameters S-(A) and S-(B), which characterize how many times the radii of gyration of blocks A and B increase, respectively, along an applied field during birefringence. The theory explains and gives quantitative results for the alignment of ordered structures and for the shift of transition temperatures.