ALMOST AS SOON AS blacks won real political power in the District of Columbia a decade ago, some began worrying that whites who had fled the city for the suburbs eventually would return to reclaim control. In this view, it was beside the point that Washington was "Chocolate City," with seven out of every 10 residents black. This theory held that whites -- particularly the Board of Trade set and the news media -- had a secret agenda for wresting control. It was known as "The Plan," and many felt it was only a matter of time before a white politician would be elected mayor and undermine much of the progress made by blacks.

In the ensuing years, the city's electorate has grown more sophisticated and discerning, yet the suspicion still lingers. Local politicians, labor leaders, academics and average residents insist that many people take "The Plan" seriously.

In their 1981 book "Perspectives of Political Power in the District of Columbia," Charles W. Harris and Alvin Thornton write that many blacks believe that at some point around the mid-1970s, whites made a decision to return to the District. "Some blacks refer to the situation as 'The Plan' -- a strategy by whites to 'repossess the city,'" they wrote. "Again, whether or not any such overt decision was made by whites in this regard, the result was the same -- a gradual uprooting of blacks, circumstantially forcing them out of the District."

"I don't think the fear of The Plan has changed a bit," says City Council member John A. Wilson (D-Ward 2), a black who represents the ward that ranges from Dupont Circle to the Southwest redevelopment district and is the city's most racially mixed. "It's an undercurrent that flows through the city. I think a lot of people fear that."

This theme is repeated time and again in conversations with the District's politicos and labor leaders, who casually treat the subject of The Plan as a given, an undisputable mindset, a political fact of life. Joslyn Williams, a black who heads the metropolitan area AFL-CIO, says he thinks the conspiracy theory is widely held. A black city council member, who declined to be identified, said that a black preacher recently approached him at a public meeting and urged him to stay in politics to help prevent whites from "taking over."

And though he may not refer to it directly, Mayor Marion Barry routinely invokes the specter of The Plan. Three weeks ago, for example, he accused The Washington Post of investigating his minority contracting program in order to destroy the program and his administration. Referring to The Plan is still a good applause line before many black audiences.

In a nutshell, The Plan is an awkward attempt to explain events that benefited whites at black expense. Belief in the The Plan is rooted in a conviction among blacks that whites are cold-blooded pragmatists who calculate their every move and who, in their heart of hearts, do not like black people.

Belief in The Plan is the legacy of slavery and segregation in this country. Yet in some ways, the worst fears of the conspiracy proponents have been realized. And surprisingly, as it becomes a less preposterous notion, The Plan has become less threatening to some blacks.

Washington's population, now 622,823, has grown smaller and whiter over the past 20 years. Between 1960 and 1980, the city lost 25,623 residents, many of them blacks forced out by urban renewal, condominium conversion and gentrification. Howard University demographer John Reid predicts that by the year 2000 the city's black population, now 70 percent, may shrink to 56 percent as blacks continue to leave the city and whites move in.

A white, David A. Clarke, was elected chairman of the D.C. City Council in 1982, a watershed election that confirms the perception that a white could be elected mayor down the road. And whites increasingly are exerting political influence. Ward 3, the stronghold of affluent whites located west of Rock Creek Park, was decisive in swinging the 1978 Democratic mayoral primary to Barry. It also provided the essential base of support last year for Republican Carol Schwartz, a former school board member who ousted at-large City Council member Jerry Moore, a Baptist minister and longtime fixture of D.C. politics.

At the same time, however, some blacks concede, almost sheepishly, that the swing back to a more socially and politically integrated city has not been all bad.

Middle-class blacks have benefited from dramatic changes in their neighborhohoods. Attractive, well-kept houses and restaurants have sprung up in once depressed areas of Northeast and Southeast Washington. An infusion of whites has rejuvenated or galvanized many advisory neighborhood commissions and groups and spurred drives to spruce up the neighborhood and upgrade city services.

As whites have moved back, police protection in changing neighborhoods has improved. The white business establishment, once viewed as a nemesis because of its tendency to go behind the city's back to cut deals with leaders on Capitol Hill, now is closely allied with local black leaders.

Across the city, from the Shaw neighborhood to areas east of the Anacostia River, the changes have not gone unnoticed. Each time someone who is white moves in, there is talk -- sometimes filled with as much anticipation as anxiety.

"We're really looking forward to the Metro subway coming out here because we know that whites are sure to follow," said James Bunn, owner of J and B's Barber Shop on Martin Luther King Avenue SE and chairman of that area's neighborhood advisory board. "With the arrival of whites, we can expect improved city services and the decline of Southeast as a forgotten area."

Along the tree-lined streets of Potomac Avenue NE, longtime Washington residents sit on their porches, actually counting the number of whites that pass by.

"They must have moved in during the middle of the night," said an amazed Beverly Thompson, a whimsical smile on her face. "I'm not saying that they sneaked in; it's just one day I looked up and there was a whole bunch of them."

A neighbor, Rhonda Ward, a District government employe, shrugged with satisfaction. "We had six children in our family and the one thing our mother kept telling us was the whites were coming back, so make sure you bought some property and keep the mortgage payments up or else we'd be swept away by the tide. She was right, and now all of us are homeowners. But a lot of people -- well, they are just gone."

Welcome as the white influx may be in some respects, blacks still cannot overcome their suspicion that they may be overpowered politically. The flash point -- where many blacks draw the line -- is the issue of control of the mayor's office, which is the most powerful political position in town and an important symbol of black dominance.

District voters, overwhelmingly Democratic and liberal in their outlook, have shown a willingness to vote for white candidates such as Betty Ann Kane for citywide seats on the city council and the school board. But the decision is much more difficult and complicated when the top political office is at stake. Blacks spent years battling for home rule and a chance to elect one of their own as mayor. Many would be unwilling to give that up, even if presented with an attractive white candidate with solid civil- rights credentials.

The mayor's office controls about 100 largely patronage city jobs, makes appointments to scores of important boards and commissions and awards city contracts -- the most tangible spoils of home rule. For the first time in this century, black and other minority businessmen by law must receive at least 35 percent of the city's contracts for goods and services. That amounted to about $158 million last year. Partially as a result, Washington has become a mecca for black entrepreneurs and professionals.

This largesse, combined with the demographic changes that are occurring, make it possible for Barry to arouse fears among his legions of supporters of a white renaissance or takeover whenever he feels his administration is being threatened. For example, a year ago during a federal grand jury investigation of drug use by city employes, Barry accused U.S. Attorney Joseph diGenova, a white Republican, of leaking information about the case to try to "lynch" Barry politically.

Blacks fears of being pilloried aren't totally groundless. Prior to home rule, the District was governed, plantation-style, by powerful Southern congressmen, such as Joel Broyhill (R-Va.) and John McMillan (D- S.C.). The underpinning of their resistance to home rule was a rock-ribbed belief -- often stated publicly -- that blacks were incapable of running a city government. The residual effects of their anti-black sentiments still linger.

The history of how blacks became politically dominant in Washington begins in the late 1950s, when blacks first constituted a majority. Drawn by relatively high-paying federal government jobs and generous municipal and social services, they migrated here from the South. At the same time, whites began leaving the District -- as whites were leaving big cities all over the country -- lured by the suburbs.

By the 1970s, all one had to do was to turn on the television set to know he was in the nation's black capital: Renee Poussaint, Max Robinson and Jim Vance, all blacks, were the big names in local TV news. The hottest radio station in the area was the Howard University-owned WHUR, which was nationally known for playing virtually all black music -- "360 degrees of soul" was its theme.

Washington's growing black constituency gave rise to local personalities like Petey Greene, a one-time inmate at the Lorton Reformatory who turned television talk show host and radio personality. Throughout the late 1960s and well into the 1970s, Greene owned the streets of Washington, merrily shouting in response to passersby who recognized him.

But a combination of factors, from rising gasoline prices to the baby-boom generation's aversion to the perceived sterility of the suburbs, made living in the District once again socially and economically attractive to whites. Former slums became targets for real-estate developers who saw the opportunity to renovate for an affluent white market.

Before his death two years ago, Greene confided that he no longer felt comfortable walking the streets. People weren't recognizing him like they used to. The problem, as he saw it, was that the lower-middle-income families -- the barbeque-rib and collard- green lovers who were the backbone of his audience -- were fast disappearing.

Blacks in staggering numbers had been forced from the city to make way for urban renewal projects in Southwest Washington, the renovation of Victorian-style row houses on Capitol Hill and east of the Anacostia River, and smart-looking, secure condominium apartments near Dupont Circle that were the trade mark of the real-estate boom of the 1970s and 1980s.

Ironically, many of these changes stemmed from the condominium conversion laws and other housing and economic policies approved by a black controlled City Council and two black mayors, Walter E. Washington and Barry. "The Plan" works in strange ways.

A study released in early 1980 by the D.C. Rental Accomodations Office showed that some 25,000 residents had been displaced during the previous five-year period, primarily because of condominium conversion, renovations and increased housing costs. The change had often pitted elderly or poor longtime black residents against young, affluent white professionals in a struggle for dilapidated houses.

As blacks left in large numbers, they were replaced by far smaller numbers of whites and, to an even lesser extent, Hispanics. John Reid, the Howard University demographer and sociologist, argues that the black population will continue to decline because of a chronic shortage of low-and moderate-income housing and a scarcity of jobs for unskilled laborers. Even if Reid's projection that the black population will fall to 56 percent within 15 years is off target -- as a population expert for the U.S. Census Bureau recently insisted -- the impact of a unified white vote in Washington nonetheless appears to be gaining in importance.

Matthew Watson, a former D.C. auditor and student of District politics, contends that because of the large number of young blacks in the city who don't vote as frequently as their elders, whites may have a slightly larger political base than the 70-30 black- white split suggests. As Watson sees it, the District may be fast approaching a balance of political power between blacks and whites in some political contests.

The election of Clarke was a major event in city politics and offers intriguing possibilities for how a white might eventually be elected mayor.

The conventional wisdom is that Clarke was only able to win the citywide contest because two well-known blacks, Arrington Dixon, the incumbent, and Sterling Tucker, a former council chairman, split the black vote. But Clarke carried six of the city's eight wards, losing only in Wards 7 and 8, east of the Anacostia, and showed impressive strength in many predominatly black precincts. He won with 40,702 votes to 25,950 for Dixon, and 24,555 for Tucker.

Clarke had something else going for him: what one of his supporters once described as a resume "that looks like a black man's resume."

In many ways Clarke is the prototype of the white candidate most likely to be elected mayor one day. He grew up in Washington, attended D.C. public schools, got his bachelor's degree at George Washington University and then obtained his law degree from Howard University. Later he became involved in the civil-rights movement, as a lawyer for the D.C. chapter of the Southern Christian Leadership Conference and as an aide to the NAACP Legal Defense Fund, and became active in the drive for statehood for the District. As a member of the city council from Ward 1, which includes Adams Morgan and Shaw, he forged strong alliances with labor, tenants' groups and black churches.

Even so, his fear that blacks would not accept a white as council chairman led him to waiver early in his 1982 drive and, for a brief time, drop out of the race. Of all the endorsements Clarke received that year, perhaps the most important one came from Bishop Walter McCollough of the United House of Prayer for All People. McCollough is the spirtual leader of several thousand city residents, but his endorsements carries extraordinary weight within the city's black community.

After the election, Tucker said that Clarke's victory had triggered "grass-roots black fears about an imminent takeover of the city government by whites." He said that Barry's apparent role in helping Clarke -- in return for white support for the mayor's reelection campaign -- "makes the theory appear complete."

"You cannot put the lid on this kind of talk," Tucker wrote in a column that appeared in The Post. "People may be wrong, but their emotions are genuine and deeply felt."

There is practically no chance anyone other than Marion Barry is going to be mayor of Washington anytime soon. He defeated incumbent Mayor Washington and Sterling Tucker in the 1978 Democratic mayoral contest largely with the support of whites in Ward 3 and a timely endorsement from his favorite whipping boy, The Post. In 1982, Barry won reelection, soundly defeating Patricia Roberts Harris in the Democratic primary. This time he did it without the support of Ward 3, where whites had become disenchanted with Barry's policies and controversial administrative style. Again, The Post endorsed Barry.

Most observers feel that Barry is unbeatable and will coast to victory for yet another term in 1986. But beyond the Barry era, the field may well be wide open, and a white politician of Clarke's stature and ties to labor and tenants groups might stand a good chance of being elected mayor.

And what would happen if a white were elected mayor? The new mayor, anxious to consolidate his support, would immediately move to appease or calm the fears of his black constituents. To stay alive politically, he or she would have to tailor programs to help blacks and keep the minority contracting program alive. Even Mayor Ed Koch of New York, who alienated blacks with insensitive comments four years ago, made an all- out effort this year to woo black voters in running for reelection.

Clarke says that race unquestionably is a factor in District politics but that race "is not an insurmountable burden" in running for mayor. "Race is a factor just like the gender of the candidate . . . . The populace of this city is conscious of race and sex, but it's more conscious of whether its trash gets picked up . . . . Everything else being equal, race has some weight. But everything isn't always equal, so the decision often is made on other issues."