Sen. Patrick J. Leahy, (D-Vt.), chairman of the Senate District Appropriations Subcommittee, asked the General Accounting Office yesterday to help him judge whether the city's proposal for a downtown convention center makes economic sense.

Leahy wrote Elmer B.Staats, comptroller general and head of GAO, asking for a detailed analysis of the $109.7 million project and an estimate of whether it would do as much business as city officials expect.

Although Leahy's request did not indicate either opposition or support, it was clear that the senator's decision may rest upon the response from GAO. He asked that the report be submitted by May 18 as a basis for hearings.

The convention center was authorized by Congress in 1972. The law requires that financing plans for the project be reviewed by the congressional appropriations committees.

Leahy, a self-described fiscal conservative, joined the Senate Appropriations Committee this year and was immediately elected chairman of its subcommittee on the District budget. As such, he may play a life-or-death role in the approval of the center.

Three years ago, a similar District proposal went unchallenged in the Senate, but was blocked by Rep. William H. Natcher (D-Ky), chairman of the House District Appropriations Subcommittee.

Natcher recently told Mayor Walter E. Washington that he now supports the center.

D.C. budget director Comer S. Coppie said yesterday that he feels the city has nothing to fear from an objective analysis by GAO.

Speaking of Leahy's letter to GAO Coppie told a reporter: "I think that's healthy and constructive. I don't see it as a threat. I see it as an effort by Sen. Leahy to meet his new responsibilities."

Mayor Washington announced the renewed proposal last Friday for an 896,500-square-foot structive just south of Mt. Vernon Square, the site of the city's old main library.

Shortly after the mayor's announcement, a rival proposal for a convention center near Largo in Prince George's County was denied financial support by the Maryland General Assembly.

In a press release yesterday, Leahy noted that the downtown center, "as typical of other civic centers around the country, is not expected to ever break even and would, therefore, require a permanent subsidy" of $9.6 million in debt serivce and up to $2.7 million in operation costs.

Leahy noted that the center's economic feasibility depends upon attracting "large numbers" of convention delegates, who would create a demand for hotel rooms, restaurant meals and other goods and services here in Washington.

Among questions he posed to GAO were those of whether there is a nationwide demand for additional convention space, whether the city's estimates of visitors are accurate, whether the costs and construction timetable are reasonable, what are the costs of pollution, congestion and dislocation in the area, and whether the convention center and the redevelopment of Pennsylvania Avenue are both needed to revitalize downtown Washington.