Thousands of urban tenants are haunted by the specter of a cold, impersonal notice that their apartment is "going condo." It means buy or get out. Other thousands already have been displaced and put to the further expense of moving from residential rental properties converted into condominium units for sale. There is no quick way to reverse the economic distortions that have generated the spread of conversions.But the resulting crisis has been unnecessary. It could be ended tomorrow.

All it would take would be for Patricia Harris, secretary of housing and urban development, to release guidelines for FHA-insured financing of low-down-payment loans enabling those same tenants to own their apartment building as cooperative housing.

Congress authorized and the president approved the program, Section 203[n] of the National Housing Act, more than four years ago. If we had it now, 203[n] would make it easy for most renters, in apartments now slated for conversion, to buy their present units for a down-payment costing little more than two to three months' rent. Usually, an extra month's rent already is available from the security deposit paid when the renter moved in.

How does the program work? Take, for example, the Chastleton, a 315-unit apartment house on the northeast corner of 16th and R streets NW. This Washington landmark is now facing conversion. It could be converted into a cooperative instead of a condominium. Combining an exishing HUB program for the blanket mortage loan -- Section 203[i] -- with individual loans for each tenant, as provided in the pending Section 203[n] program, the renter of the average one-bedroom apartment in the Chastleton would need only a $600 down-payment on a $16,600 purchase price as a cooperative owner.

In sharp contrast, when the same $16,600 apartment is converted to a conventionally financed condominium, the downpayment is likely to be at least $3,000. Clearly, the 203[n] program would provide the most affordable alternative to renters caught unprepared by a conversion notice.

the availability of Section 203[n] financing would bring significant benefits to the landlord, too. For one thing, each tenant would be enabled to buy as a result of the low down-payment, thus avoiding displacement. Also, studies show that in many areas, because of inflation, in as few as four months the actual co-op monthly costs turn out to be less than existing rents.

District of Columbia law provides for the tenants to have a right to buy an available rental property from the owner. The supporting infrastructure, outside of government, has been in place and waiting. HUD's four-year delay amounts to an executive-agency veto of a law passed by the Congress.

HUD has kept city officials in the dark about the consumer advantages -- or even the existence of low-downpayment cooperative housing legislation. In such an information void, it is unfortunate but understandable that the D.C. Council's recent moratorium on condo conversions also has blocked the benefits of 203[n] co-op conversions -- even if HUD were to make the plan available now.

Before the moratorium is reconsidered this fall, Mayor Marion Barry and the council should consider cooperative housing not as part of the problem but as part of the solution. Meanwhile, Secretary Harris is holding the key.