Voltage stability imposes important limitations on the power systems operation. Adequate voltage stability margin needs to be obtained through the appropriate scheduling of the reactive power resources. The main countermeasures against voltage instability could be distinctly classified into preventive and corrective control actions. This paper proposes a preventive countermeasure to improve the voltage stability margin through the management of the reactive power and its reserve. The voltage and reactive power management is studied from the generator’s point of view to maximize effective generator reactive power reserve (EGRPR). Detailed model of the generators including the armature and field current limits, as well as the switch mode between the voltage control and the reactive power limitations are considered to maximize the reactive power capability of the generators in emergency states. One-stage and two-stage optimization approaches are utilized to find the optimum solution. The proposed optimization procedure is applied on a 6-bus system and the New England 39-bus system to illustrate the effectiveness of the method.