When a layer of a semicrystalline polymer is subject to a tensile force in its plane and a voltage through its thickness, the deformation of the layer is initially homogeneous, but it then localizes. The electromechanical instability sets in when the force and the voltage reach critical conditions. The critical conditions are determined in this paper and are related to the following two special cases: the Consid̀re condition for the necking instability and the Stark-Garton condition for the pull-in instability. The general critical conditions show that a tensile force can markedly reduce the critical voltage. © 2009 American Institute of Physics.