MOST OF THE ADVOCATES of a downtown convention center have not exactly welcomed local business and civic leader John W. Hechinger's offer to build a privately financed, city-leased facility on his lumber company's land in Northeast Washington. Although the city's Mount Vernon Square plan in stalled on Capitol Hill, its champions are resisting any talk of compromise and fear that Mr. Hechinger's eleventh-hour proposal may undermine their stand. Indeed it may - but some rethinking might be constructive, in our view, because Sen. Patrick J. Leahy (D-Vt.) seems adamantly opposed to the downtown plan in its current form. And while Mr. Hechinger's idea may not be a good answer, it does at least illuminate some questions for review.

Instead of a totally public project, Mr. Hechinger is suggesting a coilaborative approach. Private investores would finance and build the center to the city's specifications; the District would promote and operate it under a long-term lease. Because the land is already assembled and would be provided rent-free, the city would save the time and the $22 million or more required for site assembly at Mount Vernon Square. Overall, Mr. Hechinger says that his plan might cost the city $6 million to $7 million in annual rent, or $1.5 million to $2 million less than the annual debt financing under the city's plan.

Until Mr. Hechinger releases more details, it's impossible to gauge the soundness of his plan or the likelihood of attracting that much private capital. It's obvious, though, that the site - the main source of the savings - is also a major problem. The company's land, at Maryland Avenue and Bladensburg Road NE, is in an unappealing area for tourists without quick Metro access to downtown. Moreover, while a center there might help revive the depressed H Street corridor, it seems unlikely to attract the concentrated private development that the Mount Vernon Square project is intended to stimulate in old downtown.

That points to two big questions: Is the downtown site really worth $22 million or more in acquisition costs? And if so, to whom? One way to get at an answer is to ask whether the same facility on the Hechinger site, or elsewhere outside downtown, would be so unappealing that big conventions would not come. If that's the case, then the $22 million may be a good public investment, perhaps vital to drawing the business to produce the tax gains to pay for the center.

But if the real concern is the revitalization of old downtown, then perhaps those who are banking on redevelopment around Mount Vernon Square should bear part of the added cost of building the center there. Perhaps the private interests that stand to benefit most directly - that is, those involved in various aspects of the tourist business - should be prepared to make a heavier investment in the project in any case. Sen. Leahy has certainly suggested this, but most local business leaders recoil from the very thought. They maintain that, for example, the kind of arrangement outlined by Mr. Hechinger may not be feasible at all, and would certainly be impossible downtown where no single developer controls the entire site.

Again, Mr. Hechinger may not have the answer. But at least he has come forth with something involving more private initiative and responsibility, and probably more private risk, than those who back the city's plan seem willing to assume. That's why shooting holes in Mr. Hechinger's plan seems to us to be an inadequate response. If the rest of the business community really wants a convention center, it's time to start thinking seriously about increasing the private commitment - or otherwise cutting the public costs.