THE DECISION BY the Federal National Mortgage Association to stop buying most mortgages in the District from banks and savings and loan firms came as a rude surprise. The mortgage market in the city is strong even with high interest rates. But FNMA's decision was not based on the real estate market. It was a result of the city's problems with its well-intentioned but useless usury rate.

The ceiling on interest for loans was raised to 15 percent by the council through emergency legislation procedures. But that procedure has been ruled illegal by a superior court judge. The ruling is being appealed. Meanwhile, FNMA does not want to be in a position of buying mortgages that might later be found to be illegal.

FNMA is not a major mortgage supplier in the Washington real estate market was seriously damaging, nonetheless. It triggered the Federal Home Loan Mortgage Corp. also to stop buying mortgages from the District. And together those firms constitute the largest buyer of mortgages and thus the largest source of mortgage money in the city. Without the regular sale of mortgages to secondary buyers like those two firms, the amount of money that banks and S&Ls have for mortgages will soon be depleted. And there will be no new source of money. Coupled with the recent slip in housing prices here, as reported by area realtors, a shortage of mortgage money could temporarily leave the real estate market flat. And that would have a terrible effect on the area's economy.

What can be done? City Councilman John A. Wilson suggests that Congress should act to waive its review period for permanent legislation raising the usury rate to 15 percent. Del. Walter E. Fauntroy (D-D.C.) and Rep. Michael Barnes (D-Md.) introduced legislation yesterday to pass over the review period and let the permanent usury law become effective immediately. Under normal circumstances, that action could take a month or more to gain approval from both houses of Congress and then from the White House. If the situation is treated as an emergency that period could be cut to about a week or two. That would be good, but it carries with it the dangerous precedent of congressional intervention in the actions of elected District officials. Another possibility would be a request to the U.S. Court of Appeals for a speedy ruling on the appeal of the superior court decision that the emergency lawmaking was illegal. Even so, a speedy ruling could simply mean a faster denial of the appeal.

The heart of the matter lies with the District's decision to maintain a usury ceiling. Maryland and Virginia have no such limits. The competitive market in home loans has worked to keep the mortgage rate in those states at the same rate it is in the District. The current usury ceiling of 15 percent in the District is protecting few people, in any case. The economies of inflation and booming housing market have teamed up to make a usury ceiling an anachronism and a disservice. It is acting to prevent District residents from obtaining legal loans. And it could press people to seek loans from loan sharks. The council should consider whether the time has not come to do away with the usury law altogether.