WHEN SEN. PATRICK LEAHY opened his press conference yesterday, he pointed out that the idea of a convention center in the nation's capital wasn't particularty new; area businessmen first sought federal backing for a center in 1840. Now, some 138 years later, that idea may become a reality. Sen. Leahy (D-Vt.), who holds most of his subcommittee's proxies on this issue, has finally approved the city's revised plan.

His decision is a welcome one. Earlier this year, he vigorously opposed the city's plan for a center, arguing that it was too large, too costly and did not include financial involvement from the business community. As chairman of the Senate Appropriations subcommittee on the District, he sent city officials back to their drawing boards to revise the proposal. His objections were taken to heart; the revised plan called for a smaller and less costly center. Local business and hotel taxes would be used to pay for the site and the cost of the center's expenses for the first five-years. And 45 letters of commitment were included from businesses interested in building projects that could bring in a substantial amount of new tax money to the city - money that would be used to cover the operating costs of the center.

Although Sen. Leahy still has strong reservations about some of the business commitments, he is persuaded that 15 of them could raise as much as $16 million - enough to meet the necessary $7.5 million in operating costs. As for his other requirements, he seems satisfied that the city has lived up to its part of the bargain, though he did suggest that the overhanging threat of a referendum on the center could turn off business supporters and turn his decision around. That possibility should not deter city officials from buying the land and starting construction. Businesses that expressed an interest in building projects should also get started; the surest way to prove the sincerity and soundness of those commitments is to begin work on them as quickly as possible.