The District of Columbia's population, continuing its downward slide, dropped by 7,000 between 1980 and 1981 to 631,000, according to new Census Bureau estimates. That's the lowest figure for the nation's capital since the late 1930s.

Although detailed reasons for the 1.1 percent decline were not available, Donald E. Starsinic, chief of state and national estimates for the Census Bureau, said it probably reflects continued movement to the suburbs of black families with children.

"At some point, the movement of families ought to stop," Starsinic said. "But it hasn't happened yet. You can see that in the very high proportion of unused seats in the D.C. public schools."

The District's official 1980 census population was 638,333.

As Washingtonians face a vote, possibly this fall, on a proposal to convert the District into a state, it's interesting to note that there are currently only four states--Vermont, Delaware, Wyoming and Alaska--with smaller populations. In 1950, when Washington's official population peaked at 802,000, the city had more people than 13 states and the then-territories of Alaska and Hawaii.

The new census report showed Maryland's population rose in the year by 47,000 to a 1981 total of 4,263,000 while Virginia gained 83,000, reaching 5,430,000. That's a continuation of the trend in the 1970s for Virginia and an upturn for Maryland.

California, the No. 1 state in population, gained 527,000 people last year for a total of 24,196,000. The industrial states clustered around the Great Lakes, except for Wisconsin, all lost population.