In this paper, a new formulation for coupled circuit-electromagnetic (EM) simulation is presented. The formulation employs full-wave integral equations to model the EM behavior of two- or three-dimensional structures while using modified nodal analysis to model circuit interactions. A coupling scheme based on charge and current continuity and potential matching, realized as a generalization of Kirchoff's voltage and current laws, ensures that the EM and circuit interactions can be formulated as a seamless system. While rigorous port models for EM structures can be obtained using the approach discussed herein, it is shown that the coupling paradigm can reveal additional details of the EM-circuit interactions and can provide a path to analysis-based design iteration.