MICKEY MOUSE and Peter Jennings will report to the same top boss. Tom and Jerry and Road Runner are teaming up with Bugs Bunny and Elmer Fudd. This news lends an air of fantasy to the latest frenzy of media mega-mergers. But if one studies the sheer throw-weight of these planned amalgamations, there is every reason to worry about the antidemocratic nature of this unopposed rise of mediarchy - and the unreassuring sense of a Clinton administration without a policy to address it. From the Sherman Antitrust Act in 1890 to the Celler-Kefauver Antimonopoly Act of 1950, Congress designed safeguards to prevent firms from merging with other firms if the effect was to substantially lessen competition and create monopolies. Trust-busting was spawned by the excesses of the Industrial Revolution, and the efforts of tycoons to corner such industrial markets as oil, sugar, steel, locomotives, and farm equipment. The order breaking up American Telephone and Telegraph in 1984 recognized that the same rationale is valid for the Communications Revolution. But the only flicker of recognition from Clinton's Justice Department is its interest in perhaps restricting Microsoft's plan for tying in access to MSN, its on-line network, for millions of buyers of Windows 95. In this information age, export of Windows 95 and thousands of hours of movie and TV content such as ''Jurassic Park'' and ''Bay Watch'' helps America's trade balance. Goods built on electrons make up one-sixth of the domestic economy. Cash cows that menace Oligopolies that seek to control both electronic content and distribution ''pipelines'' are cash cows. But they also menace democracy. They have enormous power to manipulate thought and emotion, woo weak wills with voyeuristic wants, and sculpt the way we look at reality. We've been slow to understand their influence in a consumption-driven world. The marriage of mediarchies involving Turner Broadcasting and Time Warner will create the globe's largest amusement-and-news enterprise. It pools Turner's Hanna-Barbera cartoon library with Warner's Looney Toons to create the world's most extensive and lucrative treasury of cartoons. It merges the interests of the two largest cable systems in the US - Time Warner cable and John Malone's Tele-Communications empire (TCI), which owns 21 percent of Turner. These two entities enter nearly half the cable homes in America. Producers who can't persuade these systems to air their programs won't make it on cable. Independent artists will confront a crisis of access. CNN and the Time family of magazines share common news audiences and cross-promotional interests. Westinghouse Electric and CBS together own 15 television stations, which deliver programs and advertising to a third of American TVs. Sumner Redstone, chairman of Viacom (with its MTV) and Paramount Pictures, also owns National Amusements Inc., which controls 1,000 movie screens, and Blockbuster, the largest renter of movies on tape. The bulk of what we see, hear, learn, sing, play, rent, and consume will shortly be controlled by a dozen corporate entities within this mediarchy. Some anticipate that economic shakeout will reduce that dozen by half by the year 2000. Buying for profit, not quality Opinion surveyors report that 80 percent of Americans learn most of what they know about the world from TV news. But we see ABC effectively capitulating to the makers of Philip Morris cigarettes on its news reports about nicotine levels in cigarettes, a matter that extends from teenage habits and health to press freedom. ABC apparently chose to clear its decks for acquisition by Disney. We're told that Disney and ABC will create a chain of ESPN sports bars, and air Disney programs in children's viewing hours, because that would ''offend no political position.'' Michael H. Jordan, chairman of Westinghouse, made it obvious that his interest in buying CBS is to acquire the stations it owns, not to revive its news division. The aim is not to serve the public interest, but to optimize profit and minimize risk. We are left totally in the dark on how President Clinton will confront the greatest trend toward monopoly in a century. In 1941, NBC was ordered to divest itself of one of its two networks, the Red or Blue, to promote diversity on the air. The ''Blue'' became ABC. Subsequently, and for much the same reasons, movie studios were ordered to sell off the movie houses they owned. MCA was told that it could not be a producer while also serving as talent agent. And ITT was kept from buying ABC because ITT's international interests were held to compromise the credibility of the network's news. Today all this case law seems to have been overlooked - though Justice reportedly is examining unspecified aspects of the Disney-ABC deal. The president faces what may be a crucial test of his place in the democratic history of this republic.