Every few years someone pipes up with the proclamation that the urban crisis has vanished.

Richard Nixon did it in 1973 when he declared, "City governments are no longer on the verge of financial catastrophe.... The hour of crisis has passed."

Harper's magazine editors did it last month when they announced in a headline, "The urban crisis leaves town and moves to the suburbs."

Such booming conclusions are at best illusory (and at worst fatuous) because they lead us to think of cities as a monolith. They are not. Some are in solid fiscal shape. Some are still in trouble.

The good news for 1979 is that many urban experts do not expect any other major city to teeter on the fiscal brink the way Cleveland, New York and Newark have done.

The bad news is that certain older cities in the Northeast and Midwest are still fiscally fragile and coule suffer severe stress if the national economy takes a dive.

"When the country comesa down with a bad economic cold, these cities come down with fiscal pneumonia," says John Shannon, assistant director of the federal Advisory Commission on Intergovernmental Relations.

Among those distressed cities are Detroit, St. Louis, Buffalo, Pittsburgh and Philadephia, he said.

Such cities, according to economist Deborah N. Matz of Congress' Joint Economic Committee, never fully recovered from the 1973-1975 recession, "and if we have another one, they'll be in even worse shape."

Ironically, this is considered a good time for cities. We can see positive signs all around us. Look at Quincy Market in Boston, Logan Circle in the District, Fells Point in Baltimore or Queen Village in Philadelphia.

Houses are being rehabilitated; businesses are moving into charming arcades and malls; whole neighborhoods are being upgraded; developers are performing plastic surgery on the faces of downtown America.

To be sure, cities have had massive injections of federal aid in recent years.

Last year the total amount of federal aid to state and local governments was $80 billion, about $30 billion of which went to localities.

But the giddy days of federal wine and reoses are over.

The cutoff of anti-recession aid, which in two years totaled $3.2 billion for state and local governments, had an extra sharp sting for a number of cities.

It forced Detroit to cut its city payroll by 348 employes; Newark dropped 441; New York decreed a hiring freeze; Chicago decided to sell off some unused city-owned land.

New Orleans was not hit directly, but a city spokesman said the cutoff "caused us to look down the road, and we saw what was happening with federal aid as a whole." So the city -- swimming against the anti-tax tide -- raised three local levies to avoid a $30 million deficit.

Donald Haider, a deputy assistant secretary of the Treasury, explained how important the antirecession money was to cities, which had depended on it to maintain services and avert layoffs withoutraisingtaxes. "It was glue money," he said. "They could use it to plug leaks. Now they're telling us, 'You've blown a hole in our budgets.'" Haide cited a new report by the Council of Economic Advisers that predicts that state and local surpluses, which dwindled rapidle late last year, will vanish completely this year.

Although the Carter administration has said it will try again to get an antirecession aid bill -- sharply reduced and directed only at cities with severe problems -- through Congress, many urban experts do not expect much new federal money for urban America.

Richard P. Nathan of the Brookings Institution here says, not at all facetiously, that "the heyday of urban policy may turn out to havebeen the period just beofore the announcement last March of the Carter urban program."

He noted that the administration backed down last year on some of its efforts to "target" funds to distressed cities and that Congress itself is leery of the idea.