Constitutive equations are derived for the viscoelastic response of amorphous polymers in the vicinity of the glass transition temperature Tg. The model is based on the theory of cooperative relaxation in a version of the concept of traps. Adjustable parameters are found by fitting data in static and dynamic tests for equilibrated polycarbonate and poly(vinyl acetate). The existence of some crossover temperature Tc is revealed at which the energy landscape becomes inhomogeneous. The temperature Tc may be treated as a counterpart of the critical temperature predicted by the mode-coupling theory for glass-forming liquids.