  An extended Nambu-Jona-Lasinio (NJL) model with chiral group $U(2)\times
U(2)$ and spin-0 and spin-1 four quark interactions is used to develop the
gauge covariant approach to the diagonalization of the $\pi-a_1$ mixing in the
presence of electroweak forces. This allows for manifestly gauge covariant
description of both the non-anomalous and anomalous parts of the effective
Lagrangian. It is shown that in the non-anomalous sector the theory is
equivalent to the standard non-covariant approach.
