  A buildup of the vertical polarization in the resonant electric dipole moment
(EDM) experiment [Y. F. Orlov, W. M. Morse, and Y. K. Semertzidis, Phys. Rev.
Lett. 96, 214802 (2006)] is affected by a horizontal electric field in the
particle rest frame oscillating at a resonant frequency. This field is defined
by the Lorentz transformation of an oscillating longitudinal electric field and
a uniform vertical magnetic one. The effect of a longitudinal electric field is
significant, while the contribution from a magnetic field caused by forced
coherent longitudinal oscillations of particles is dominant. The effect of
electric field on the spin dynamics was not taken into account in previous
calculations. This effect is considerable and leads to decreasing the EDM
effect for the deuteron and increasing it for the proton. The formula for
resonance strengths in the EDM experiment has been derived. The spin dynamics
has been calculated.
