(FORTUNE Magazine) – THE U.S. Department of Transportation is about to become a big player in antitrust policy. Last year the department assumed responsibility for + approving airline mergers when the Civil Aeronautics Board flew out of existence. Now the department must rule on three proposed airline combinations -- Texas Air and Eastern, Northwest and Republic, and TWA and Ozark. Says an airline lobbyist: ''There's a strong possibility that Transportation will bust up one or two of them.'' Transportation Secretary Elizabeth H. Dole will make the decision, and she has been getting plenty of advice from interested parties. This spring the Justice Department, usually permissive in antitrust matters, urged Dole to turn down the Northwest-Republic merger, arguing that such a combination would hurt passengers. A squadron of airline labor unions and competing carriers have attacked the Texas Air-Eastern deal. At the House Judiciary Committee, which held hearings on the latest round of airline combinations, Chairman Peter W. Rodino Jr. warns: ''Undue concentration could undermine competition to the point that deregulation is doomed to failure.'' The Texas Air-Eastern marriage has encountered the most turbulence. Competitors charge that a combination of Eastern and New York Air, a Texas Air subsidiary, would lock up the lucrative Northeast market. The two airlines operate over 95% of flights between New York's La Guardia airport and Washington's National. Says Harold J. Pareti, chief of Washington-based Presidential Airways: ''The benefits of competition must be preserved.'' USAir, American, and Delta say they are eager to offer service between these airports, provided Secretary Dole forces Texas Air to give up some slots now held by Eastern or New York Air. (Slots are takeoff and landing rights at airports where the number of flights is restricted.) Texas Air says competitors have enough slots to begin service between the two Northeastern cities but use the slots to fly other places instead. Dole is likely to reject that argument and approve the acquisition only if Texas Air gives up some slots. The main objection to the Northwest-Republic combination is that the two Twin Cities competitors now handle about 71% of the traffic to and from the Minneapolis-St. Paul airport. The Department of Justice says competitors are not likely to expand service greatly in Minneapolis because of its extreme northern location, so most passengers would have to fly with the combined Northwest-Republic. Northwest needs Republic's domestic network so that it can feed more passengers to its Far Eastern flights. Unless it merges with Republic or another carrier, Northwest will have a tough time against giant United, which acquired Pan Am's routes to the Orient late last year. Airline lobyists set the odds of Dole approving the Northwest-Republic deal at about fifty-fifty. Both TWA and Ozark have hubs at St. Louis's Lambert International Airport, where together they handle about 74% of the flights. However, other carriers -- such as Republic and Southwest -- could take advantage of St. Louis's middle-of-the-country location and provide plenty of competition for TWA- Ozark. Antitrust experts say this deal should have little problem getting off the ground.