THIS MORNING THE SUPPORTERS of a downtown convention center have their second chance to present the best case for this $109.6-million project to Congress. The Senate appropriations subcommittee hearings two weeks ago had been considered a crucial test because Chairman Patrick Leahy (D-Vt.) had been so skeptical about the project's feasibility. But those hearings were disrupted by senatorial scheduling conflicts and Mr. Leahy's squabbling with Mayor Walter Washington and other witnesses. Testimony for and against the center was cut short. Not only was Mr. Leahy left skeptical: the best arguments pro and con were not even fully laid out - except in the written record, which will probably be read by only a few senatorial aides.

We hope a more coherent debate will develop at Rep. William Natcher's hearing today. Though Mr. Natcher has been far more sympathetic toward the project, the veteran House chairman has never been known as an easy touch. To get his support for the first $27.7 million in city borrowing, the center's advocates will have to persuade him that their calculations are sound and that the District can really carry out such a huge development program efficiently.

Ironically, the best argument may be the "worst case" - the worst that failure could bring. The District government and the business community have not tried this tack. They have emphasized the public benefits they foresee up to 390,000 new convention goers a year, around 4,000 local jobs, net revenue gains of at least $12 million annually, revival of the Mount Vernon Square area. They have not talked much about what the city and its taxpayers might be saddled with if those forecasts are wrong. But that "worst case" might not be quite as catastrophic as the center's opponents charge. For instance, suppose the center winds up costing $120 million instead of $109.6 million. According to the city, the added debt burden would not wipe out anticipated revenue gains. Or suppose that only 300,000 more convention delegates show up, and half of them go to the expanded Sheraton-Park facilities. The convention center might still generate enough tax revenues to cover its bonds.

Those are the kinds of figures that Congress and the community need before final decisions are made. Obviously, some crucial factors cannot be calibrated in advance; the first of these in the city's capability to manage the center competitively and to orchestrate private development. But if the city and its allies talk more candidly about the risks, as well as the blessings, which can be measured now, those who are still uncertain about this project - including ourselves - might have a bit more faith in it.