A method for making a solid catalyst component for use in a Ziegler-Natta catalyst includes combining a porous particulate support with a magnesium source in a hydrocarbon solvent to form a mixture, the magnesium source including a hydrocarbon soluble organomagnesium compound and a hydrocarbon insoluble anhydrous inorganic magnesium-halogen compound. The organomagnesium compound is halogenated and the mixture is reacted with a titanium compound or vanadium compound to form the solid catalyst component. The solid catalyst component is then recovered and combined with an organoaluminum cocatalyst to form a Ziegler-Natta catalyst which is advantageously used for the polymerization of olefins, particularly alk-1-enes such as ethylene, propylene, 1-butene, and the like. The catalyst can optionally include internal and external electron donors.