  Nonperturbative methods have been well-developed for QED with the so-called
t-electric potential steps. In this case a calculation technique is based on
the existence of specific exact solutions (in and out solutions) of the Dirac
equation. However, there are only few cases when such solutions are known.
Here, we demonstrate that for t-electric potential steps slowly varying with
time there exist physically reasonable approximations that maintain the
nonperturbative character of QED calculations even in the absence of the exact
solutions. Defining the slowly varying regime in general terms, we can observe
a universal character of vacuum effects caused by a strong electric field. In
the present article, we find universal approximate representations for the
total density of created pairs and vacuum mean values of the current density
and energy-momentum tensor that hold true for arbitrary t-electric potential
steps slowly varying with time. These representations do not require knowledge
of the corresponding solutions of the Dirac equation, they have a form of
simple functionals of a given slowly varying electric field. We establish
relations of these representations with leading terms of the derivative
expansion approximation. These results allow one to formulate some
semiclassical approximations that are not restricted by the smallness of
differential mean numbers of created pairs.
