WHAT BEGAN as one senator's terrible idea is gathering menacing steam on Capitol Hill. It's a proposal guaranteed to make National Airport noisier, more congested and more subject to the special interests of Congress. This week it managed to win the approval of a committee that should know better. Chief sponsor of the measure is Sen. John McCain, Republican of Arizona, which is the home state of America West, an airline hungry for a bigger piece of the Washington market -- and never mind what the National traffic will bear.

The bill is being sold in the name of competition, but inquiring senators will discover they're being sold a bill of dangerous goods. The legislation would eliminate federal limits on the number of flights an hour at National as well as at New York's Kennedy International and LaGuardia airports and Chicago's O'Hare International. For more than 20 years, the Federal Aviation Administration has limited the number of scheduled passenger jet flights at these heavily used airports to reduce excessive delays. At National the limit is 37 flights an hour -- no small number at a terminal that size.

Opponents of the McCain bill, including members of the regional airports authority that Congress approved to run National and Dulles, point out that the measure could create chaos at National. They note that a $735 million renovation underway there could be inadequate if traffic is increased. Although many airline officials relish the prospect of wide-open entry at National, at least two companies have recognized the real effects of rush hour in the skies overhead: Trump and Pan Am shuttle representatives have said that without the FAA's "slot" landing-rights system, they may not be able to guarantee hourly arrivals in New York and Washington.

Right now -- thanks to that same regional authority, approved in the name of better airport management -- National Airport is being rebuilt into a new, safer and more suitable terminal closer to Metro and with more parking. There is no good reason to wreck this work. Any senator concerned about the effects of congestion, noise or the agreement under which National and Dulles are being renovated should vote to reject this bill in its entirety.