  We show in this paper how to construct Symanzik polynomials and the Schwinger
parametric representation of Feynman amplitudes for gauge theories in an
unspecified covariant gauge. The complete Mellin representation of such
amplitudes is then established in terms of invariants (squared sums of external
momenta and squared masses). From the scaling of the invariants by a parameter
we extend for the present situation a theorem on asymptotic expansions,
previously proven for the case of scalar field theories, valid for both
ultraviolet and infrared behaviors of Feynman amplitudes.
