In recent decades psychologists have shown that the standard model of individual choice is often violated as choices are influenced by the decision context. We propose that voting behavior may be similarly influenced and we introduce a theory of context-dependent voting. Context-dependence implies that preferences over any pair of alternatives may depend not just on the two options but on the entire choice set. With an analysis of data gathered during the 1996 U.S. congressional election we confirm the presence of a significant context-dependent effect on voting behavior. In addition, we demonstrate that, when applied to a simple, standard model of electoral competition, context-dependent voting yields an equilibrium in which only two candidates compete and adopt divergent policy platforms, thereby deterring additional entry. The equilibrium is simultaneously consistent with policy divergence and the stability of two-party political systems that underlies Duverger's Law.