The five Democratic members of the Fairfax County Board of Supervisors last week jointly endorsed President Carter for reelection.

Announcement was made by the vice chairman of the board, Martha V. Pennino (Centreville), who praised Carter's commitment to local government. Joining Pennino were supervisors Sandra L. Duckworth (Mount Vernon), James M. Scott (D-Providence) Joseph Alexander (D-Lee) and Audrey Moore (D-Annandale).

The four Republican members of the board have not publicly endorsed a candidate and say they are not likely to do so.

Chairman John F. Herrity, a Republican, said last week, "I get enough flack for everything else I do without endorsing somebody."

But in an apparent attempt to see that no one would get confused about his political intentions, Herrity added, "I'm a member of the ABCK club. Do you know what that stands for? Anybody but Carter or Kennedy."

Another Republican, Thomas M. Davis Ill (Mason), says he has thrown his support to Ronald Reagan.

The other two Republican board members, Marie B. Travesky (Springfield) and Nancy K. Falck (Dranesville), have followed their fellow GOP board members and have refrained from any public endorsements. But both were seen wearing "Bush for President" buttons at a recent meetings of the county board.