John G. Milliken, 34, former aide to Rep. Joseph L. Fisher (D-Va.) will seek the Democratic nomination for a seat on the Arlington County Board, Milliken said yesterday.

Milliken, a Washington attorney, said he plans to announce his candidacy at a meeting of the Arlington Democratic Committee tomorrow night.

Milliken's intention to run as a Democrat rather than as an independent with endorsements by the Democratic Party and the ostensibly nonpartisan -- but overwhelmingly Democratic -- Arlingtonians for a Better County (ABC) organization signals a possible departure from the county's recent tradition of nonpartisan elections.

Since 1974, both Democratic and Republican candidates for the county board have run as independents with party endorsements because of the Hatch Act, which bars Arlington's large population of federal employes from participating in partisan campaigns.

ABC leaders have said outright nomination of a candidate would preclude those workers from participation in local politics and could mean the end of the 25-year-old organization, one of the oldest and most effective reform movements on the East Coast.

Democratic leaders said privately yesterday that ABC's recent losses to Republican board candidates means that the party, whose only function is to work for the election of county board candidates, has lost its clout. Furthermore, they say voters would be more likely to vote the straight party ticket in November.

A decision about whether to nominate a Democrat or endorse an independent candidate will be made at a May 10 mass meeting, Milliken said.