We present a nonlinear model of an electrically actuated microbeam-based resonant microsensor encompassing the electrostatic force of an air gap capacitor, the restoring force of the microbeam, and the axial load applied to the microbeam. The model accounts for moderately large deflections, dynamic loads, and the coupling between the mechanical and electrical forces. It accounts for the nonlinearity in the elastic restoring forces and the electric forces. A perturbation method, the method of multiple scales, is applied to the distributed-parameter system to study the local dynamics of the sensor under primary, superharmonic, and subharmonic excitations. In each case, we obtain two first-order nonlinear ordinary-differential equations that describe the modulation of the amplitude and phase of the response and its stability, and hence the bifurcations of the response. The perturbation results are validated by comparing them to experimental results. The DC electrostatic load affects the qualitative and quantitative nature of the frequency-response curves, resulting in either a softening or a hardening behavior. The results also show that an inaccurate representation of the system nonlinearities may lead to a qualitatively and quantitatively erroneous prediction of the frequency-response curves. The results provide an analytical tool to predict a microsensor response to primary, superharmonic, and subharmonic excitations, specifically the locations of sudden jumps and regions of hysteretic behavior allowing designers to examine the impact of the design parameters on the device behavior.