THE MUSEUM of the City of Washington?

Not yet, though the City Museum Project Inc., organized since 1976 to promote the idea, has produced its first exhibit.

"Two Centuries of Change: The Idea of Downtown Washington" is open now through Oct. 13 at the National Portrait Gallery's Great Hall. After that, the show will move around to Washington neighborhoods for another year and a half. Earl D. James, chairman of the City Museum Project Inc., and the 250-member organization hope that the exhibit will arouse interest in the history of the city and a museum to display it.

The exhibit was designed and installed by Root and Chester Design consultants. The history of downtown Washington is told in 193 photographs with informative captions. Interspersed are blowups of important people of the city's history. The exhibit includes many of black Washingtonians, emphasizing their contributions to the city. The pictures are reproduced from the great collections of the Columbia Historical Society, that unsung center of scholarship on the city; the Library of Congress, the Martin Luther King Washingtonian room and private collections.

The exhibit defines the traditional downtown as bounded by Third Street on the east, the Mall on the south, 15th Street on the West and K Steet on the north.

The show centers on the ordinary working people of the town-- where they lived, shopped, worshiped and worked. It's not a show about presidents or palaces. It tells about the workaday places and the people who went about their business between the parades and the inaugurations.

The City Museum had hoped to open a year from now in a 2,000-square-foot space in the old Lansburgh Building. But now that the General Services Administration has pulled the National Archives out of the building, a new sponsor will have to be found.

The City Museum Project was organized 3 1/2 years ago. As yet, it has no collection of its own. The National Endowment for the Humanities put up $59,000 for planning and production of this exhibit. Downtown's Lowdown

Washington's downtown, besides that of New York City, or even Philadelphia, is low profile, low rise and low budget. The nongovernment downtown -- that area on the "wrong" side of Pennsylvania Avenue, where the town folk shop and are entertained -- has too often been either cursed or ignored.

Thanks to the views of Thomas Jefferson and others, the city has never aspired to great heights of commercial development. The height limit of 40 and now 60 feet in some areas has never favored a concentration of skyscrapers. And just as well, considering that Washington is built on a soggy marsh land, laced with underground rivers.

Even so, the commercial buildings sometimes seem poor stepsisters to the marble monumentality of the wealthy side of the family, the federal buildings. The most interesting of the old commercial buildings too often have been the first to be razed by those bent on "improvement." The Willard Hotel, one of the prettist on F Street, narrowly escaped. Lansburgh's hovers in a sort of purgatory. Kann's already has been lost to fire. The old Masonic Lodge, later Julius Lansburgh's furniture store, stands on the edge of the abyss.

But pleasant, even homey, buildings and streets still stand in Washington's downtown. The low buildings allow the sun to touch all the corners, unlike the dark caverns of New York. The frightening whirlwinds and eddies caused by the skyscrapers elsewhere are here only friendly breezes. The buildings are on a human scale, not so big that they make you shake in your boots while you crane your neck to see the tops. Fortunately, there's no attempt at uniformity, as in museum cities such as Williamsburg. Downtown Washington represents all its history, though often hiding its past under layers of remodelings.

The scale is not only manageable up but also across. You can pick up some money at American Security bank at one end and walk the whole length of F and G streets, spending money all the way, till you have to use your charge card when you get to Hecht's at the end of the street.

Along the streets are hotels, bookstores and big department stores such as Woodward & Lothrop's; such fascinating upstairs stores as the G Street Remnant Shop; ice cream stands; health food stores; the Martin Luther King Library (Ludwig Mies van der Rohe's great design); enough shoe stores to handle an army, and dozens of places to buy dresses, not to memtion the marvelous Janus-faced museums at the end: the National Collection of Fine Arts and the Portrait Gallery.

If at the end a rather tawdry plaza rather resembles a shipwreck, well, it does offer a few places to sit while you eat those good donuts from the ship in the same block. Upstairs, artists lean out their windows, and they might even sketch you! Picturing Downtown

The City Museum exhibit and catalogue is welcome, to remind us of where we've come from, in time for us to ponder where we're going.

The exhibit begins in 1803, when houses, shops and offices were all pleasnatly tumbled together in the midst of the wide fields. There was no uptown or downtown and hardly any town at all.

Zoning was not a common idea. Back then it seemed sensible to put the stores where the houses were, so people could walk to buy bread or ribbons. Offices were often above the shops, and living quarters above those. Who, after all, in his or her right mind would want to drive an hour to get to work or buy hamburger?

The perils of Washington in the 1800s were many. Cows, chickens and goats were always nudging their way through the gates and out to the street. The frame buildings, often hastily constructed, caught fire with regularity.

Some glimpses from the exhibit's photographs and captions, written and researched by Marcia Greenlee, historian and curator, with Robert Mawson as research assistant:

Blodgett's Hotel at 8th and E Streets, (shown in an 1803 photograph) was both patent office and post office until it burned in 1836. Next door was Joseph Gales, publisher of the National Intelligencer. People must have felt they were living in the country.

John Quincy Adams in 1820 bought the duplex house at 13th and F Streets shown in the 1865 photograph. It was thought to be quite fancy. Houses later were converted into shops before being razed at the turn of the century.

Maryland and Virginia had the largest concentration of blacks in the States in 1791, when the District of Columbia was laid out. Alethia Tanner, shown in a picture, was so successful at growing vegetables near Lafayette Square that she bought her own freedom in 1810 and eventually that of her family.

It wasn't easy for blacks even after they had bought their freedom. At a challenge, they had to show certificates of freedom, such as the one shown for Eliza Washington. They were restricted by other laws, as well. Though the city was a center of slave trade until 1850, the free black population outnumbered slaves. An antislavery poster of 1837 in the exhibit shows slave prisons in downtown Washington.

In the mid-19th century, various ethnic groups settled here. Chinese brought their New Year's festivals with the dancing dragons. Greeks settled near John Marshall Place and Pennsylvania Avenue. Germans organized their Washington Saengerbund. Each group had its own church: Adas Israel Synagogue (1876), New York Presbyterian Church (1803), St. Mary's Catholic Parish (1848) for German speakers.

Some businesses went back to the town's first days. Publishing was always a big employer here, and the National Intelligencer, shown at 7th and D Streets about 1860, was one of the most important. The editor, William W. Seaton (portrayed about 1860), also was mayor from 1840 to 1850. Newspaper row was on 14th Street, north of Pennsylvania Avenue, close to the Western Union telegraph office.

Not for many years did senators and representatives and their followers buy houses here. The large transit population lived in boarding houses or hotels. Some of the best known are shown in the exhibit: Rhodes Tavern (photographed in 1815 and again in 1912, when it had become a drugstore) at 15th and F; Brown's Indian Queen Hotel (1832) on Pennsylvania Avenue; James T. Wormley's (1871) at 1500 H Street; the Willard Hotel (1880), 14th and F Streets; the Ebbit House Hotel at F and 14th Streets.

The stores came along in a hurry. Woodward & Lothrop came to F Street in 1887. The Hecht Co. opened in 1896. And to supply the wherewithal, the banks moved in fast. The Riggs opened in 1880 at 15th and Pennsylvania Avenue. The Capital Savings Bank, opening in 1888, was the first black bank in Washington. The American Security and Trust Co., shown in 1901, had a department just for women, as did the Riggs as late as 1956.

Flower vendors flourished in 1870. The Center Market, with its 1,000 stalls, was called the Marsh market because of its soggy location.

The change in shopping habits can be charted in the exhibit photographs. During World War II, people stood in line to buy shoes. As late as the '40s, trolleys provided an easy way to get around town. Window shoppers in the '40s longed for the new television sets.

Entertainment was a first priority. The new National Theater, the fifth on the site, is shown in 1910. A poster, touting Edwin Booth, is from 1956. Vaudeville and movies were popular between 1910 and 1940. Sightseeing, a major Washington entertainment, is represented by a splendid photograph. Ladies with full white skirts, high neck blouses and marvelous feather hats, with their menfolk in bow ties and straw hats, are all ready to ride the "Seeing Washington" open-air wagon.

Downtown always needed improvement. In 1871 a great festival celebrated the new wood-block pavement for Pennsylvania Avenue.

A whole section of pictures in the show portrays crises in Washington. Troops marched down F Street during the Civil War. The great flood of 1880 washed away lives and livelihoods. The Bonus Army of 1932 camped here and demonstrated on Pennsylvania Avenue. The 1963 demonstration for civil rights led by Dr. Martin Luther King was one of the biggest. Looking for History

When the City Museum Project began to research the show, "the first thing we found out," said James, "is that no history exclusively dedicated to downtown Washington exists."

It's not as though there has been no scholarship work done on the city. The Columbia Historical Society periodically publishes fascinating studies of the city. Recently, there have been two excellent and comprehensive works on the city.

James Goode's monumental "Capital Losses" has just been published by the Smithsonian Institution. Both residential and commercial buildings are covered, but it concentrates on those that have been torn down. Pictures from his book are currently on exhibit at The Octagon through October 13.

The Commission of Fine Arts recently has published another remarkable work of scholarship documenting 16th Street's historical district, following its earlier work on Massachusetts Avenue and Georgetown. But the Commission hasn't gotten around to documenting the downtown.

Perhaps the only study concentrating exclusively on the downtown, besides the City Museum effort, is by Linda Wheeler, a Washington Post writer and photographer. She has just completed "Washington, D.C., the Old City," a photo documentary on Washington's downtown. The book focuses on the people and buildings being displaced by the convention center and other current big building projects. She hasn't found a publisher yet.

Washington's monuments and memorials, the Federal City, have been celebrated in book and exhibit, but Washington's own citizens' center has rarely been noticed.

But now, signs are everywhere of a revival of interest in the downtown. The gas shortage has sparked a move back to the city as a place to live and a place to shop. Some of the downtown department stores with their big spaces and more extensive stocks have larger volumes than their suburban sisters. New buildings, hotels, offices and stores are going up. The Sunday subway has already tripled expectations of riders and shoppers. Perhaps before too long, we'll even have a City Museum in our rediscovered downtown.

New life and interest are riding the subway to the downtown.