A microwave imaging approach based on the integration of a subspace-base optimization method (SOM) and a multi-focusing procedure is proposed. The scattering equations are expressed within the contrast source formulation of the electromagnetic inverse problem and then solved by a nested procedure which includes an outer multi-resolution loop (dealing with the identification of the regions of interest), a spectrum analysis step (devoted to the reconstruction of the `deterministic' components of the contrast sources) and an inner optimization loop (aimed at the retrieval of their `ambiguous' parts). A numerical analysis is presented to preliminarily assess the features of the proposed approach.