Sen. Patrick Leahy (D-Vt.) is expected to announce today his long awaited decision on whether the District government can go ahead with plans to build a downtown convention center.

One prominent business leader who supports construction of the $99 million civic center said Leahy, chairman of the Senate D.C. Appropriations Subcommittee, told him last week, "I don't think you'll be unhappy" with his decision.

Leahy has scheduled a noon press conference today to announce whether he will approve scaled-down plans for the project, planned near Mount Vernon Square.

Leahy has virtual one-man control over the decision because he controls a majority of the proxies on the subcommitte, which must approve the center if money appropriated for it in District's 1978 fiscal budget is to be released.

Opponents of the center said last night they are prepared to lose this round, but praised Leahy for delaying his decision long enough for them to organize a referendum drive that could force the question on the ballot next spring.

Leahy, who would not tip his hand yesterday about his decision, appeared to enjoy the suspense he had created by inviting both foes and supporters of the project to separate meetings in his office before the press conference.

Opponents were invited to meet with him at the Russell Senate Office Building at 9:45 a.m. and business leaders who support the center were told to be there at 11 a.m.

John H. Phelan, an opponent active in a petition drive, said "it doesn't matter" what Leahy decides today.

"If he approves it, we can beat it at the polls. If he disapproves, city officials will be back next year reintroducing it. It's up to us to beat it. If we don't, we've got no complaint."

Carol Currie Gidley, another opponent, said hundreds of signatures already have been secured on petitions being circulated in all eight wards of the city, and they expect to get the minimum 12,476 names as voters leave the polls Tuesday.

The initiative referendum, if approved, would bar the mayor or city council from committing any more public funds to the project. The 1978 city budget included $27 million for site acquisition.

Leahy balked at approving the original plans, which called for a $116 million center. After the size and cost of the center were revised downward, Leahy held hearings last month at which he challenged backers to prove that construction of the center would spark enough related development to produce at least the $7 million in annual taxes needed to pay off building costs.