The new president of the University of the District of Columbia sought congressional support yesterday for a revised expansion program -- including development of all four square blocks of UDC's Mount Vernon Square property and construction of a third campus in Southeast -- despite a steadily declining enrollment.

Rafael L. Cortada, who took over as president Oct. 1, told a House District of Columbia subcommittee that UDC has been unable to operate effectively or attract students because of crowding at its Van Ness campus and at other facilities it rents throughout the city.

"No other institution is told to generate the programs and enrollment prior to being given the facilities in which to do this," Cortada told the subcommittee on judiciary and education. "Our egg cannot lay a chicken."

In outlining new plans for the Mount Vernon Square site between Seventh and Ninth streets near the Washington Convention Center in downtown, Cortada said the campus would include a proposed two-year community college, the Center for Graduate Studies and Research, the School of Business and Public Management, the Miner/Wilson School of Education and Human Ecology, and a replacement for the Antioch Law School.

He said a third campus also is needed in Southeast Washington -- possibly on 80 acres at the city-operated St. Elizabeths Hospital for the mentally ill -- to reach a large segment of the city's black population that is physically cut off from UDC's main Van Ness campus on upper Connecticut Avenue NW.

"If the university is to be equally accessible to all District residents, it must begin to explore establishing a presence on the east side of the Anacostia River," he said.

Cortada and N. Joyce Payne, chairman of the UDC Board of Trustees, told the subcommittee they favor scrapping the university's five-year-old strategy of seeking private financing for the Mount Vernon Square campus by opening a portion of the site to commercial and residential development.

Congress at one time had authorized spending $40 million to develop the Mount Vernon Square site, but denied UDC those funds in 1980 after enrollment began to slide. Enrollment throughout the university has dropped by one-third in the past eight years, from about 15,000 to less than 10,000.

Rep. Mervyn M. Dymally (D-Calif.), chairman of the subcommittee, urged UDC and other city officials to move quickly on the revised proposal and predicted that Congress would respond favorably.

"It's clear the university will never be able to complete its mission without consolidation of the physical facilities," Dymally said.

Del. Walter E. Fauntroy (D-D.C.) also urged District officials to resist the temptation to sell off a portion of the Mount Vernon site, prime real estate near the Washington Convention Center, to help finance university construction.

The UDC Board of Trustees is expected to consider Cortada's proposals at its November meeting. However, Mayor Marion Barry's administration indicated yesterday that it is in no hurry to commit itself to the Mount Vernon Square proposal without assurances of federal support and a better understanding of UDC's long-term plans and enrollment projections.

Dwight S. Cropp, the mayor's director of intergovernmental relations, said the mayor is anxious to reduce UDC's annual rental costs, which totaled about $34 million in the past decade, by building a new campus.

City officials say that the Mount Vernon Square project could cost $100 million or more, most of which would have to be financed with city borrowing even if Congress reauthorizes the $40 million.