Recent experiments have disclosed electrical effects well ahead of an advancing shock front. Certain of these effects have been attributed to the diffusion of electrons through the shock front from the ionized region behind it. The diffusion hypothesis is examined in terms of a simple heuristic model, in which the diffusion is assumed to take place from a plane electron source moving with the shock velocity. According to this model, there is no true electron ``front,'' as has been suggested by some experiments with ionizing shocks. However, a transient in the electron distribution may give rise to a virtual front in certain experimental situations. Under reasonable assumptions, the existing observations of precursor fronts by Weymann are found to be consistent with the predictions of the model.