We study a lattice gas with repulsive nearest-neighbor interactions driven to steady state by an external electric field E. Using Monte Carlo techniques on a two-dimensional system, we find, in the E-T plane, a line of second-order transitions joining a line of first-order ones, at a point which is probably tricritical. From a field theoretical model, we show that the operator associated with E is naively irrelevant for critical behavior. This expectation is borne out by the Monte Carlo result β=(1/8.