ARLINGTON COUNTY'S ballot includes a lively, important contest for the county board between two thoughtful candidates who have run vigorous issue-centered campaigns. Incumbent Stephen H. Detweiler, now serving an unusual second consecutive year as chairman, is seeking a second four-year board term as a Republican-endorsed independent. Mary Margaret Whipple, a former member of the county school board, hopes to tip the partisan balance on the five-member county board to the Democrats. Mr. Detweiler has a record of independent thinking and moderation that stands him in good stead as the pivotal vote. We think he deserves reelection.

Mrs. Whipple argues that a change in the board majority would mean better "control" of development in the county, a greater commitment to day care, recreation and shelter facilities and other human services programs, and more money for schools. While all this has evident appeal, revenue constraints make the decisions more difficult -- and it is here that Mr. Detweiler has shown thoughtfulness on a case-by-case basis.

Careful development of the Metro corridor from Rosslyn to Ballston not only makes sense but requires realistic negotiation with developers. Trade- offs do, and should, include agreements on height and density in exchange for green space, wider sidewalks and other critical amenities. These improvements, as well as continued growth in established commercial areas, can at once save the government money and increase revenue yields.

At the same time, Mr. Detweiler has not harped on development as a replacement for necessary tax increases. Nor has he joined those in his party who would cut taxes and spur development to make up any difference. On the contrary, Mr. Detweiler parted with the other Republicans to vote for a necessary increase in the property taxes.

Though we differ with Mr. Detweiler's opposition to a ballot proposal for a redevelopment and housing authority, voters have an opportunity to address that question separately, and the entire county board will deal with it accordingly.

Mrs. Whipple has presented a good challenge that could be even more valuable in another year, against another candidate. But Mr. Detweiler's role as a moderate and mediator on this board should not be lost to the county.