Sen. Patrick J. Leahy (D-Vt.), whose appropriations subcommittee shares life-or-death power over the D.C. government's proposal to build a $110-million downtown convention center, said yesterday he is "not convinced" the project can pay its way without long-term tax subsidies.

Leahy, in his first year as chairman of the Senate District Appropriations Subcommittee, made the comment after presiding over two days of hearings during which the project was strongly supported by District officials and business spokesmen but criticized by several witnesses representing citizen groups.

The District has asked Congress to approve an initial loan of $27.7 million from the U.S. Treasury to buy land and start designing the center. The proposed exhibition hall would occupy a three-block site south of New York Avenue between 9th and 11th Streets NW.

In practice, the decision on the funds likely will come from Leahy's subcommittee and its House counterpart, headed by Rep. William H. Natcher (D-Ky.). Natcher has endorsed the project, while Leahy remains openly skeptical.

Under the District's limited form of home rule, Congress still must approve the city's annual budgets. The convention center loan is an item in the 1978 fiscal year's budget, which is now pending. No congressional action is expected for several weeks.

Leahy's views were apparent as he questioned supporters of the project and as he responded yesterday to its critics. He agreed with some who contended their opposition had not been taken seriously enough by the D.C. City Council.

After yesterday's session, Leahy told a reporter he wants "to read over all the testimony and take it up with the rest of the (sub) committee" before making a decision. Then he added:

"So far I'm not convinced it's economically feasible - that there has been a commitment by anybody else but the taxpayer, in an involuntary fashion . . . It may well be a good idea for the city to have a convention center, but that doesn't necessarily mean it's going to make money."

Leahy voiced fear the center might require long-term subsidies from city taxes - District government officials have consistently maintained the city will benefit financially if the center is built.

Ben W. Gilbert, D.C. municipal planning director, said he and other city officials "were prepared to answer the senator's understandable questions fully at the hearing" Wednesday, but time was cut short.

"We will provide the answers in writing, and we are confident our answers will confirm our case," Gilbert said.

Foster Shannon, a real estate executive and president of the Metropolitan Washington Board of Trade, said that if Leahy "will study the expert reports and look what's happened in other cities (that built convention centers), he'll make an affirmative judgment."

As the hearings opened Wednesday, District officials, led by Mayor Walter E. Washington, contended that the center will draw between 310,000 and 390,000 convention visitors each year, eventually producing an indirect profit to the city treasury and several side benefits - including 3,000 new hotel rooms and 4,000 jobs.

City Council Chairman Sterling Tucker said the project represents a reasonable economic risk for the city and promises renewal of a decaying neighborhood. "The question should not be whether to proceed, but how - and when," Tucker told Leahy.

Executives of two national hotel chains, Jim Durbin of Bethesda-based Marriott Hotels, Inc., and John Nordlander of Minneapolis-based Radisson Hotel Corp., said during the hearings that their firms would seriously consider building hotels near the convention center once the center's construction is assured.

Both hotel men appeared with a panel of center supporters assembled by the Board of Trade. Questioned by Leahy, Durbin said Marriott would proceed if an independent economic analysis showed the new hotel would average at least 80 per cent occupancy year-around.

Yesterday, as the second day of hearings opened, Leahy said Marriott's position was far from a firm enough commitment to convince him. "Looking at the feasibility is a far sight from promising to build a hotel," he commented.

Shannon, the Board of Trade official, said the hotel men "went as far as they could possibly go at this point. These people don't make idle chatter."

Joseph B. Danzansky, chairman of a Board of Trade committee promoting the center, pointed to the willingness of the business community to continue an income surtax to help support the center in early stages of operation as evidence of its commitment to the project. Washington hotels also have agreed to impose an 80-cents-a-day 'convention center room tax.'

With time running out on Wednesday, opponents representing chiefly citizen groups were heard briefly. Those unable to speak before adjournment were invited to return yesterday.

John D. Kelly, representing the Upper Northeast Coordinating Council, noted widespread opposition by citizen coalitions and predicted burdens on taxpayers.

"We taxpayers are to take the risk, the gamble for this thing," Adeline Bickerdyke of the Capitol Hill Southeast Citizens Association said.

J. George Frain of the Kalorama Citizens Association said a government-owned site on Pennsylvania Avenue NW should be used instead of the Mount Vernon Square site, which the city will have to buy for more than $25 million from private owners.

Not all citizen testimony was critical. Harrison Lee, president of the Chinese Consolidated Benevolent Association, who often is called "the mayor of Chinatown," supported the center. "It would be replacing an area which has decayed," he said.