Members of the D.C. Democratic State Committee joined five Democrats on the City Council yesterday in endorsing Statehood Party candidate Hilda Mason to fill a vacant at-large City Council seat in the July 19 special election.

Yesterday's endorsement by the group, called Democrats for Hilda Mason, was the first time that the top four officers of the District's Democratic Party and about 15 members of the state committee publicly endorsed Mason.

Also endorsing Mason were City Council Chairman Sterling Tucker, School Board President Therman Evans, and teachers union president William Simons.

Recently, the state Democratic Committee decided not to endorse any candidate in the special election because some members of the committee were opposed in principle to the party's endorsing a non-Democrat and others were opposed because they believe the party's funds should be used to build the party and not to endorse a non-Democrat.

City law prohibits on party holding more than two at-large seats on the Council. Democrats already have two at-large Council members. Those who endorsed Mason yesterday said that while the party would not endorse any candidate they saw no problems in endorsing a candidate personally.

Those endorsing Mason include Robert B. Washington Jr., chairman of the state Democratic Party; Lillian Sedgwick, vice chairman; Inez Cain, corresponding secretary; James Nutall, recording secretary, and Barbara Clark, alternate national committeewoman, and John W. Hechinger, national committeeman.