Elastomeric seismic isolation bearings are subjected to large axial loads and lateral displacements during strong earthquakes. The existing Koh-Kelly model for elastomeric bearings accounts for axial load effects on horizontal stiffness. This linear model is based on small displacements and rotations and predicts stable postcritical behavior or increasing critical load with increasing horizontal displacement; however, unstable postcritical behavior is observed in the bearing test results presented in this study. The analytical model developed in this study, based on the Koh-Kelly model, includes large displacements, large rotations, and nonlinearity of rubber, and it predicts unstable postcritical behavior. The formulation of the analytical model, calibration, and verification using experimental results are presented. It is shown that: (1) the critical load reduces with increasing horizontal displacement; and (2) the horizontal stiffness reduces with increasing horizontal displacement and axial load. It is also shown that the critical load capacity at a horizontal displacement equal to the width of the bearing is not equal to zero, as predicted by the approximate procedure used in design, but higher.