The Census Bureau has projected that the District of Columbia's population could drop by 260,000 people by the year 2000 if the trends of the 1970s continue, but the city's chief statistician and a local demographer yesterday sharply disputed the prediction.

In its first population projections based on the 1980 census, the Census Bureau said the city's 1980 population of 637,700 could dip to 501,500 by 1990 and to 376,500 in 2000 if people continue to leave the nation's capital at the same pace they did in the 1970s.

The city's population is now larger than that of four states, but it would be smaller than that of all 50 states by 1990 if the bureau's local and national projections prove accurate.

However, both Albert Mindlin, the city's chief statistician, and George Grier, a longtime Washington area demographer, said Washington's substantial population losses of the 1970s have slowed markedly since the April 1980 census.

Mindlin said the city's planners now project a population of 638,100 in 1990 and a small decrease to 632,900 by 2000.

Grier, without projecting the actual population figures, said he thinks the District's population in the next 17 years will show either "a slight further loss or a moderate gain," but nothing akin to the Census Bureau projections.

"It's just not going to happen," Grier said. "The District has lost just about all of the population it's going to lose. No trend lasts forever."

He said that during the 1970s there was only a slight decrease in the number of households in the District. But Grier said there was a "very pronounced turnover from families with children to singles and childless couples."

He said the changeover in the District's population has largely "been accomplished" and that now "some of those couples will have children and some of the single people will get married," adding to the District's population.

Mindlin noted that the Census Bureau's own statistics show that in the two years after the 1980 census the District's population dropped by an average of less than one-half of 1 percent each year, far less than the 2.34 percent annual drop projected by the Census Bureau for the 1980s as a whole.

"They simply projected the 1970s data into the 1980s and 1990s," Mindlin said, while the "net out-migration from the city has decreased dramatically."

Signe I. Wetrogan, the author of the Census Bureau's projections, said that "it appears the District's population decline has slowed," but said that it would not necessarily be accurate to assume that the 1981-82 trend would continue throughout the 1980s.

However, she acknowledged that basing 1990 and 2000 projections on the 1970s information is "very hazardous for those places that had extreme trends, such as the District of Columbia" and would probably produce "unrealistic results."

The Census Bureau, again using the 1970s information, projected that both Virginia and Maryland will continue to gain population in the 1980s and 1990s.

Virginia's population would increase by more than one million, to 6.4 million by the year 2000, while Maryland's would grow from 4.2 million to 4.6 million, the report said.

Nationally, the Census Bureau projected that California will continue to be the nation's most populous state in the year 2000, but it predicted that Texas and Florida will move into second and third places, ahead of New York, which now is second.