THURSDAY the Senate took time out from discussing affairs of state to worry about how its members -- poor darlings -- get to the airport. That's the only interpretation that can be made of the discussion that preceded the decision of the world's greatest deliberative body to block the proposed new rules for National Airport. The Senate wants one of its committees to investigate those rules. A majority of its members are apparently afraid the rules will put a crimp in the ease with which they and their constituents fly into the center of the nation's capital.

The rules are worth worrying about, but for different reasons. Unlike many members of the Senate (and the House), we think the FAA's proposal is entirely too generous in the number of flights and passengers it will permit into National Airport every day. And, also unlike those members, we do not think the new rules will result in a shift from National to Dulles of flights serving smaller cities in the East, South and Midwest; we suspect the new rules mean those flights will either shift to Baltimore-Washington International or simply be canceled out.

Someday, whether under the proposed rules, the existing ones or others that Congress may dream up, the saturation point at National will be reached. When that happens, the odds are the big airlines will begin to replace flights from cities like Dayton, Indianapolis and Charlotte with more profitable flights from Chicago, New York and Boston. If the airlines want to continue those flights (and they might well decide to hand them off to commuter airlines), they will have to choose between Dulles and BWI. Delta, Eastern, Ozark and Piedmont have already chosen. They have pulled all their operations out of Dulles.

The fact is that Dulles is a failing airport. While business has been booming at National and doing quite well at BWI, it has been decreasing at Dulles. Dulles is losing not only airlines but passengers, flights and cities that those flights serve. It offered more daily flights to more destinations a decade ago than it does now.

Most airport owners would be worried about that, especially if their airport were still losing money 20 years after they spent millions of dollars building it and providing it with a 13-mile-long, four-lane, divided, private driveway. But the owner of Dulles -- Congress -- doesn't seem to care. Instead of urging the FAA to adopt policies that would make Dulles not only profitable but the leading jetport it was designed to be, Congress keeps pushing the FAA toward rebuilding and expanding National so that more and more people and airplanes can be jammed through the Potomac River Valley. In the meantime, the state of Maryland is doing its best to promote BWI and cripple Dulles by such actions as subsidizing bus service and creating a transportation link with Amtrak.

Instead of worrying about whether the FAA's proposed rules will curtail flights from National to their favorite small cities, senators and representatives should be worrying about whether those rules adequate to prevent Dulles from becoming the world's most beautiful white elephant. Do those rules do enough to start now the inevitable process of forcing flights and passengers out of National while there is still a chance they might be transferred to Dulles? We think they don't.

If Congress wants to tinker with the rules, as it apparently does, it should do so with an eye on scaling National down and improving ground transportation to Dulles.