This paper proposes a normative criterion for voting rules under Knightian uncertainty about individuals' preferences to characterize a weighted majority rule (WMR). This criterion, which is referred to as robustness, stresses the significance of responsiveness: the probability that the social outcome coincides with the realized individual preferences. A voting rule is said to be robust if, for any probability distribution of preferences, the responsiveness of at least one voter is greater than one-half. The main result of this paper establishes that a voting rule is robust if and only if it is a WMR without any ties. Robustness is a stronger requirement than weak efficiency because a voting rule is weakly efficient if and only if it is a WMR in which ties are allowed with an arbitrary tie-breaking rule.