DALLAS — Four key members of Congress say all airlines, not just low-fare carriers, should be able to bid on gates and landing rights that American Airlines and US Airways will give up after they merge.The leaders of the House and Senate transportation committees say they’re worried that unless the big airlines can bid, service between Washington and some smaller cities may be lost. This month, the Justice Department settled a lawsuit challenging the merger after American and US Airways agreed to give up gates and landing rights at several big airports, notably Washington’s Reagan National. Officials said those assets will go to low-cost airlines because the big airlines — led by Delta and United — had stifled competition.On Friday, top Democrats and Republicans on the transportation committees released a letter that they sent to Attorney General Eric Holder urging that bidding be open to all airlines. They said low-cost carriers don’t generally fly to smaller cities, so freezing out the big airlines won’t help consumers in those places. It was signed by Sens. Jay Rockefeller and John Thune, and Reps. Bill Shuster and Nick Rahal.The settlement was viewed as likely to benefit Southwest Airlines and JetBlue Airways, low-cost carriers that have indicated interest in getting some of the landing rights at Reagan National. Delta also expressed interest in picking up landing rights at Reagan National, plus two gates that American agreed to surrender at Dallas Love Field.“We do believe that all airlines should have an opportunity to bid on the divested assets,” Delta spokesman Trebor Banstetter said Friday. Southwest spokesman Brad Hawkins responded that legacy airlines already dominate Reagan and New York’s LaGuardia airports. Letting them bid on these gates and slots “defeats the very purpose of the divestiture — to create lower fares, more competition, and better flight options for consumers,” he said.