Down on New York Avenue, near the flophouses and the porno shops, the site of the District's new convention center was a scene of unrelieved bleakness in the rain. The steel girders thrust starkly upward from the deserted, muddy excavation, and in the desolation the whole project looked like the boondoggle its critics have always said it would be.

Three blocks away, at the International Inn on Thomas Circle, the center looked much more promising to the bankers, business leaders, lawyers and restaurateurs gathered amidst the clink of ice cubes in cocktail glasses to greet the center's new general manager, George W. Demarest Jr.

The atmosphere was one of unrestrained optimism, and the early scorecard on the center reinforced it:

According to Robert Gordon of the D.C. Department of General Services, who is project manager for the $98.7 million project, construction is "on schedule and within budget." Barring strikes or a stretch of unusually bad weather, he said, the building should be completed on schedule in August 1982.

The Center already has more than 90 conventions and trade shows definitely or tentatively booked through 1992.

Although the blocks immediately adjacent to the center's site are still an eyesore, the rest of old downtown -- from 15th Street to North Capitol Street -- is in the midst of a development boom that city officials have been dreaming of and planning for since the 1968 riots.Land prices have skyrocketed, and the city's long-term policy of using its power and the public's money to stimulate private development is bearing fruit.

Those were some of the most encouraging signs of welcome for Demarest, 47 and the former chairman of the Greater Cincinnati Convention and Visitors Bureau, when he began work here last week. He is being paid $75,000 a year, a salary that makes him the highest paid local official in the Washington area.

Demarest's arrival signals a new and decisive phase for the center, a phase that will determine whether it will attract the crowds of free-spending visitors its promoters promised or turn out to be the extravagant money-loser its opponents predicted.

The financially pressed city government is looking eagery ahead to the projected revenue from sales, liquor, food entertainment and hotel taxes that convention visitors bring, and Demarest thus holds one of the keys to the District's future financial health.

Twenty conventions and trade shows have made firm bookings and more than 70 others have tentatively reserved dates in the center, a sprawling building that contains a series of connected halls, meeting rooms and retail space that can be rented in whole or part.

Demarest said he was "very pleased" with the bookings arranged by the Washington Convention and Visitors Bureau because "the groups that have conventions big enough to use a facility like this make their commitments five or six years ahead and so there's always a lag in the early years of any new facility."

The early optimisim on the center could be misleading in the long run, however.

It is far from certain that bookings at the center will ever reach the figures advanced by its promoters to justify the expense of building it: 320,000 new convention delegates attracted by 31 large conventions a year, four or five major trade shows a year with 13,000 participants each, and at least six other smaller gatherings each year.

Few of the convention centers that have proliferated in depressed downtowns all across the country have approached those figures, and the construction of major new centers in Baltimore, New York, San Francisco, Atlanta and other cities has increased the already fierce competition that Washington faces for convention business.

Moreover, hotel executives say the District does not yet have enough hotel rooms to handle a really major convention, the kind that requires 12,000 rooms or more.

While new hotels that are under construction or planned downtown will add more than 3,500 new rooms to the city's current stock of about 30,000, consultants who specialize in hotel planning say it is still possible that any such gathering would fill up hotel rooms in Virginia and Maryland, defeating the purpose of a facility built to bring jobs and revenue to the city.

Nor can the sudden surge of office and hotel development in Washington's once-depressed downtown -- a phenomenon that convention center boosters say already guarantees enough of an increase in District property tax revenue to cover the cost of building the facility -- be entirely attributed to the center.

The opening of Metro, the work of the Pennsylvania Avenue Development Corporation and the shortage of land west of 15th Street have spurred development east of 15th Street at least as much as the center has, according to planning experts.

Nevertheless, supporters of the project say the early prognosis is clearly favorable. The purpose of the center, which is rising on a 9.7 acre site bounded by Ninth, H, and 11th streets and New York Avenue NW, is to bolster the city's economy by attracting conventions previously unable to come here because of a lack of facilities, thus stimulating hotel and restaurant employment and raising enough tax revenues to justify the expense of the project.

"Properly managed, it will meet those objectives," said Austin Kinney, executive director of the Washington Convention and Vistors Bureau.

Kinney, a strong supporter of the facility, said his roster of convention center bookins validates his projections. Considering that the center is not yet completed and in fact still faces a rear-guard court challenge from opponents demanding that the project be put to referendum, Kinney said in an interview that the number of bookings is "very encouraging."

His list of "definite" bookings includes the American Dental Association convention in 1988, The U.S. League of Savings Associations meeting in 1984, the World Congress of Cardiology in 1986, the International Conference on Technical Cooperation in World Communications in 1985 and the American Heart Association Convention in 1988.

The tentative bookings include labor unions, fraternal organizations, trade associations and church groups.

Kinney stressed that the list does not inclue any rock shows, concerts, or sports events and said the cewnter is not diverting events away from Capital Centre in Landover. One of the cost-cutting measures adopted by convention planners, moreover, was elimanation of a sports arena in the facility.

The long-range booking policy will have to be established by Demarest and the center's board of directors, but Kinney, Edward A. Singletary, chairman of hte center's board of directors, and District government planning officials all stressed that the purpose of the facility was to attract out-of-town visitors, not local audiences.

The prospect of spinoff development around the center is welcome news for cash-strapped city officials like Mayor Marion Barry -- even if the rosy picture that was painted to persuade Congress to release funds for the center is never fully realized.

"It's part of an overall strategy and it's working," said John Fondersmith, chief of the downtown section in the city's Office of Planning and Development. "Guys that would not invest their money in that area a couple of years ago are falling all over themselves to do it. To get the center near Metro, we had to pay a higher price for the land than we would have elsewhere. But it looks like a pretty good deal right now."