A BACK-DOOR RAID is being made on the fledgling University of the District of Columbia, the city's one public institution of higher education, and it should be smartly repulsed. The raid began with a General Accounting Office study of projections of the likely size of UDC in future years. There is room for legitimate argument about the precise numbers, but what the GOA did was wrong. Settling ona projection lower than the UDC's own, it took the unwarranted next step of suggesting that the new college's campus should not be in Mt. Vernon Square, the area long reserved for it. Rather, said the GAO, the university should set up in the old D.C. Teachers College facilities and in assorted buildings downtwon. Sen. Patrick Leahy (D-Vt.), from his key Appropriations Committee outpost, followed by, in effect, seconding the motion to steal the preferred site from UDC.

This is unacceptable. To argue about student projections is all very well, but to consign the new university to inadequate, dispersed, transportation-poor quarters lacking in amenities and, yes, in prestige is an affront this city cannot accept. The students UDC is meant to serve will not be attracted by the plant that the senator and the GAO have in mind. Let Mr. Leahy move the University of Vermont to the second floor over the grange hall and a couple of odd cow barns and then come back to Washington with his advice.

There is, however, a point buried in this charade. It is that to plan a university plant, you must know what kind of university you are trying to build: What are its student constituency's needs and what is being offered and planned to serve them? If you intend to prepare only PhDs and "supergrades," for instance the university will be an elite place, and its physical requirements will be relatively small. If - and this is far more likely here - you intend to prepare a range of students, many of intellectual competence and many of inadequate preparation - for a broad range of careers in government and the private economy of the metropolis, then the physical requirements will be larger. As it happens, this difficult but central question is being worked out right now by the UDC Board of Trustees.Its recommendations are due shortly.

To us, it makes sense to set aside the GAO-Leahy maneuver to steal UDC's site, to reveive and debate the UDC trustees' report on the university and to get on promptly with building the facility the city's aspiring students deserve - at Mt. Vernon Square.