  A path integral representation of the evolution operator for the
four-dimensional Dirac equation is proposed. A quadratic form of the canonical
momenta regularizes the original representation of the path integral in the
electron phase space. This regularization allows to obtain a representation of
the path integral over trajectories in the configuration space, i.e. in the
Minkowsky space. This form of the path integral is useful for the formulation
of perturbation theory in an external electromagnetic field.
