  We present an effective action approach for the problem of Coulomb blocking
of tunneling. The method is applied to the ``strong coupling'' problem arising
near zero bias, where perturbation theory diverges. By a semiclassical
argument, we obtain electrodynamics in imaginary time, and express the anomaly
through exact conductivity of the system $\sigma(\omega, q)$ and exact
interaction. The calculation is tested by comparing with the known perturbation
theory result for the diffusive anomaly. Also, we use the method to study the
anomaly enhancement due to external magnetic field, and the effect of screening
by electrodes.
