Two at-large seats on the D.C. City Council will be filled by this year's elections, but the city's home rule charter permits only one of them to be occupied by a Democrat.

Whoever wins from a field of nine candidates on the Democratic ballot in the Sept. 12 primary is virtually assured of victory in the Nov. 7 general election. The seat is being vacated by Douglas E. Moore, who is seeking the Democratic nomination for council chairman.

The other seat to be filled this year will go to a member of a minority party or an independent.

Incumbent Hilda Mason is running unopposed on the Statehood Party primary ballot, as is challenger Rosenblatt on the U.S. Labor Party ballot. There is one announced independent candidate, the Rev. Warren A. Hemphill Sr., who is not involved in the primaries and whose name will go directly onto the November ballot if his petitions qualify.

The council is composed of 13 members - four at-large members, representing citywide interests; eight members representing geographic wards, and the chairman, elected at large. Of these seats, a total of seven (two at-large, three from Wards 1, 5, 6 and the chairman) will be filled this year.