Mayor Marion Barry and Sen. Patrick J. Leahy have begun 11th-hour maneuvers in an effort to forestall almost certain death in the next two weeks for a politically touchy proposal to build a new $56.7 million downtown campus for the University of the District of Columbia.

Congress already has appropriated money for the project, to be built just north of Mount Vernon Square.

But for two years, Leahy (D-Vt.), chairman of the Senate District Appropriations Subcommittee, has refused to release the funds. He is at odds with city officials, school administrators and student leaders about the need for a second school campus.

Leahy met last week with Barry to discuss several possible compromises that could break the deadlock. These included extending the deadline for spending the funds for up to a year, or selling the current Van Ness campus on upper Connecticut Avenue NW -- where Leahy has urged locating the entire school -- and then building the university entirely on the downtown site.

Both Leahy and Barry said yesterday, that no agreement had been reached.

"I'm in support of both campuses. The problem is that we don't make the decisions," Barry said. "I don't support one campus. (But) I may have to if there are no options. The decisions is in his hands."

Leahy said he was not sure whether he would ask for an extension at a scheduled meeting today of the Senate Appropriations Committee. And even if he does, he said, there was "only a 50-50" chance that such an extension would be approved.

Leahy repeated his contention that under no circumstances would Congress approve a two-site plan. "There are no more than four or five votes for that," he said.

The maneuvering on the brink of the funding deadline came as university students, who have been critical of both Barry and Leahy, began intensifying their efforts to have the money released.

Yesterday, about 20 student leaders met with Barry for more than an hour at the major's office in the District Building. Barry emerged with the student leaders to declare a unity of purpose and a common sense of persecution.

"I think what this whole thing demonstrates is our powerlesness. We need more self-determination. We need more input into our lives," the mayor said, as students applauded his remarks at the impromptu press conference outside the mayor's office.

After the press conference, Abdul-Raheem Abdullah, president of the UDC student government association, was asked if he were satisfied with the mayor's responses and Abdullah answered, "No. I'm not satisfied. I'm not satisfied at all."

Abdullah said he had received conflicting information from Barry and an aide to Leahy about a possible agreement between the mayor and the senator to support a ine-site plan. "We can't say [the aide] lied. We can't say the mayor lied. But someone is giving out conflicting information," Abdullah said.

Leahy has opposed construction of the second campus, first on grounds that the university may be building more facilities than are justified by its enrollment projections, and later on the contention that it would be more efficient to consolidate school operations at the Van Ness campus.

City officials, university administrators and students disagree. They say that the Van Ness location, in a predominantly white residential neighborhood on upper Connecticut Avenue NW, discourages many poor black Washington residents from enrolling -- especially those living in the distant far northeast and southeast parts of the city.

The present downtown campus consists of rented space in 14 buildings, much of which used to be part of the old Federal City College. FCC, D.C. Teachers College and Washington Technical Institute were merged in 1977 to form UDC.

Seven buildings are under construction at the Van Ness campus now at a cost of $51.7 million. It is the most modern and centralized campus of the system largely because when it was WTI, then president Cleveland L. Dennard, pursued an aggressive development policy.

The Van Ness site covers nearly 22 acres. An aide to Leahy said he would "sure be surprised" if a buyer could be found for the property. Any further development in the area also would be likely to meet with stiff opposition from neighborhood groups.