In cities across the United States, a cluster of ornate structures, including a town hall, a church or two, a post office and a few temple-like bank buildings, serve to transform a few urban blocks into a community's civic heart.

Increasingly, however, such bank buildings -- often classical in style with stone pillars, vaulted interiors and rich detailing that convey a sense of stability and security -- may be an endangered species, according to a growing number of historic preservationists across the country.

With a number of banks and savings and loan institutions folding nationwide, and thousands of properties falling into the government's hands through foreclosure, an untold number of fine old structures may be in jeopardy, the preservationists said.

In the District of Columbia, for example, preservationists are undertaking a survey of all financial institution structures built before 1960. Already, they have identified as many as 150 buildings, including some that have served as commercial landmarks for decades.

"We know a lot of bank buildings are going to be changing in their use," said architect David Maloney of the District's Historic Preservation Division. He said he is particularly concerned about "very fine buildings" outside the downtown area that could face possible demolition if they cease to be used as banks.

"I think from the point of view of historic buildings, banks are among the most important, and the most interesting, architecture that we have," Maloney said. "The bank is typically the fanciest building in a neighborhood."

The future of a few District banks is particularly uncertain at this point. Three architecturally rich former branch locations of the National Bank of Washington, the District's oldest bank which folded last August, recently have been put up for sale by federal regulators. Riggs National Corp. took over NBW's deposits and accounts and a few of its locations when NBW failed, but declined to take possession of most of its branch offices.

The most striking of the three structures is the old NBW headquarters at 14th and G streets NW, with its limestone facade and Roman-style columns. The lobby is particularly dramatic, with its mahogany and walnut walls, marble floors, elaborate molded plaster ceiling and enormous wrought-iron chandelier.

The building won national historic-landmark status last year -- one month before the bank folded. National historic landmark designation provides a measure of protection against a building's destruction or alteration. The designation calls attention to the site's importance in national history and encourages preservation support.

According to John Pace, the Federal Deposit Insurance Corp. official charged with overseeing disposition of NBW's real estate, the former NBW headquarters, worth about $12.5 million, is most likely to be incorporated into a redevelopment of the site.

"We think that building will sell to somebody who will build over it or around it, and will use {the whole building} as a lobby," Pace said. "It's so pretty it can't be disturbed."

Other distinctive NBW branches up for sale are at 2000 Martin Luther King Jr. Ave. SE, which Pace said is believed to be worth about $365,000, and at 800 H St. NE, worth about $575,000.

The issue of bank buildings outliving their owners is expected to become increasingly acute as the number of viable financial institutions dwindles. A decade ago, there were 4,000 savings and loans nationwide, but about 2,000 are expected to remain alive this year after regulators finish off those already considered among the "walking dead."

Meanwhile, in the past few years, more than 1,200 banks have failed, and about 200 more are expected to collapse in 1991. That would be about one-tenth of the total when the closures began.

Additional contractions within the banking industry appear likely. Some analysts said the savings and loan industry eventually may disappear, while many banks, under severe competitive pressures, may have to merge to survive.

The bank buildings themselves are of less concern in other parts of the country. Elsewhere, according to historic preservationists, the real problem is the massive amount of real estate that is falling into the hands of financial institutions, which then fail themselves. Those properties now are in the hands of federal agencies, or the property management companies they have designated. In some cases, preservationists claim, federal officials have proved more interested in selling the properties to recoup some of the losses than in preserving the national heritage.

In Dallas, a set of 14 turn-of-the-century warehouse buildings just north of the city's "West End" historic district has been marketed as a "tear-down" development opportunity site, preservationists said.

"It is really, literally, just north of the historic West End district," said Ron Emrich, historic preservation officer for Dallas, who so far has been unable to persuade federal authorities to save some of the warehouse buildings. "They are clearly eligible to be listed in the national historic register like the West End is."

The preservation of historic buildings within the government's widespread real estate holdings is a growing national concern, said Andrea Ferster, assistant general counsel of the National Trust for Historic Preservation.

The National Trust recently raised the issue in testimony before the task force overseeing the Resolution Trust Corp., which oversees the remnants left behind by failed savings and loan institutions.

"... This country will bear the scars of the savings and loan crisis for years to come, both in terms of our financial well-being and the public confidence in our regulatory institutions," Ferster told the task force. "It is critical that this harm not be compounded by failing to preserve, wherever possible, historic places as well as the communities of which they are a part."

Here in Washington, the financial institution survey being undertaken by the District is expected to be completed in the spring. Funded through matching grants from local preservationists and the National Park Service, its supporters hope it will be the first phase in a research effort that could add new insight into the financial development of the city over the years.

The researchers already have discovered an interesting pattern in the ebb and flow of financial institutions over time, according to Judy Robinson, whose consulting company is conducting the survey.

Over the years, between 50 and 75 bank buildings in the District have been demolished, she said. The most dramatic upheaval was during the Depression, Robinson said, when 13 of the District's 33 banks went out of business.

Some bank buildings, however, have managed to survive in unusual and innovative forms, with some becoming nightclubs or fast-food restaurants. Historic preservationists urged that this practice, known as "adaptive reuse," be encouraged if the structure has architectural or historic significance.

Other banks switched ownership over time, Robinson said.

Of the three architecturally significant NBW branches in the FDIC's hands, for example, all were built to house other banks. The building at 2000 Martin Luther King Ave. SE was built as the Anacostia Bank, while the structure at 800 H St. NE was built for the Northeast Savings Bank. And the grand old NBW headquarters building was originally built to house the Federal-American National Bank.

"We're of a generation that expects banks to be stable institutions," Robinson said. "Everything about their style suggests stability and security to their patrons."

The reality, based on the research her firm has conducted, is that change has been the only constant, she said.

"The banking industry through time has changed a lot," Robinson said. "Over the years, there has been a constant flow of buyouts, mergers and closures. It will be interesting to see how what happens now compares to what happened, say, in the 1930s."