THE CONTROVERSY over Washington's proposed $110 million downtown convention center appears headed for a familiar solution: While Congress and the city debate to a deadlock, the needed facility will be built in the suburbs. But this need not be.

Although a convention center in the Mt. Vernon Square area would be an important boost to the economic well-being of downtown, failure to win congressional approval for the project could provide the opportunity to create a better alternative - and simultaneously to bring new life to two civic white elephants: Robert F. Kennedy Stadium and the D.C. Armory.

For about $25 million, the stadium could be covered, the armony could be refurbished and the two could be connected by an enclosed hall to provide a total exhibit area of more than 400,000 square feet. This would be one-third more space than the Mt. Vernon Square proposal, at less than one-fourth the cost.

Both the stadium and the armory are now underused . Barring a return of major league baseball, RFK Stadium is now filled about 10 times a year for Redskin football games, with another 20 or so uses a year for college and high school football, professional soccer and rock concerts. In 1976 the stadium lost about $860,000; it faces some major repair expenditures, and its $20 million bonds will mature in 1979. The armory, although not losing money because it does not support a bond issue, is losing shows to the newer Capital Centre.

The ideal of enclosing the stadium has been around a long time, because it makes a better stadium; with so little stadium use, however, there has been little incentive. It could be enclosed by a series of suspension tent structures like the enclosure at the Munich Olympic stadiums or with an air-supported structure like the one used recently to enclose a stadium in Pontiac, Mich.

However, to put Washington in the avant-garde, and because it fits the stadium a lot better, it might be wiser to use a lighter-than-air structure - the world's first "floating roof." A network of helium -filled bags, tied and restrained like other air structures, but using blimp technology, may be the way to avoid any disruption to Redskin seats (our sacred trust) by supporting columns. It would also be cheaper. A conventional suspension roof or air-supported coverings might add $10 million in cost, taking the total project into the $35 million range.

All estimates - $110 million for Mt. Vernon, or $25 million for the stadium/armory - need to be taken with a good deal of salt. What is significant is the relationship between the two figures and, even more, the natural advantage on which the lower figure is based: free, government-owned land and reusable existing buildings.

One absolute requirement for the convention center is a flat floor exhibition area of at least 300,000 square feet. While the stadium only contains 150,000 square feet in the available flat playing field area, and the armory nets another 67,000 square feet, linking them together by a 430-foot-long exhibit hall would yield over 400,000 square feet of total floor space. Even when the temporary floor over the playing field in the stadium was removed for Redskin, Diplomat or possibly professional baseball games, the center would still have 250,000 square feet available for convention use.

Moreover, important side benefits unavailable at the downtown site or in any new facility would be the bonus of existing seating areas for over 50,000 people, column-free space and ceiling heights up to 150 feet. Access by Metro

THE PERCEIVED remoteness of the stadium/armory site remains the only vexing problem, and it may be somewhat exaggerated. Unlike the proposed Mt. Vernon Center, the stadium/armory is directly served by Metro, and part of the plan would be to enclose the station as part of the complex. Express trains from downtown could get there in 7 minutes, the local in 11 minutes. In addition, Washington's only inner-city freeway, the Southwest Expressway, goes right to the stadium, with a ramp now only used for Redskin games. If that ramp could be opened to serve only the Stadium Convention Center, a trip from downtown by car or what most convention cities use, shuttle bus, would take about 10 minutes.

A major point often overlooked is that Washington is basically a sprawling, polynuclear city and a part of a sprawling, polynuclear megalopolis, which means that a lot of people would be coming from all over by car. A large, deliberately avoided problem with the Mt. Vernon site is its complete rejection of automobile patronage. As a planner, I've come reluctantly to the conclusion that except for going to work, one is better off not trying to separate an American from his car for any trip involving choice. A convention center is a volume operation needing a site to handle volume. The stadium, with its open lands, superb access and tons of parking, is just that Mt. Vernon Square actually is the exact opposite.

What we want downtown are the hotels, bars and stores, not necessarily the convention center itself. This is a matter of policy and zoning. One could draw a very large radius around the center and prohibit hotels, while finding a carrot to develop them downtown.

In a way, we are recapitulating a previous experience here, with some of the same people now on different sides. There was once a strong effort to get the Kennedy Center built downtown as an economic catalyst. It was only later that it was realized that the difference in land costs between downtown and along the river was the difference between the project being acceptable or unacceptable to Congress. If we had been strong and inflexible enough, we would have gotten no Kennedy Center at all.

Even with indifferent architecture, it has been a boon for Washington.

Reducing the cost to one-quarter may guarantee that the center is not a financial boondoggle, as many citizens fear, and giving the stadium/armory a full time job may remove our already existing financial boondoggle. It's a compromise, but a heck of a trade. CAPTION:

Picture 1, no caption, Drawings by Arthur Cotton Moore/Associates; Picture 2, no caption