General Motors is getting tough with the major television networks. After deciding not to buy ads in next year's Super Bowl telecast, the automaker is now demanding that each of the networks cut their advertising rates for the so-called upfront period when ad buyers make the bulk of their purchases, Advertising Age reported today (Monday). The trade publication cited one ad-buying exec with the company as saying that GM could be seeking cuts of as much as 20 percent but that the networks are refusing to negotiate any price decrease at all. General Motors' stance, Ad Age observed, could "cause ripples" since it is the third-largest ad buyer in the U.S., behind only Procter & Gamble and AT&T. The magazine quoted one ad-buying executive as saying that the fear is growing among media outlets "that if one TV network deals with GM, The Others will have to capitulate."