Betty Tatum, a 46-year-old Democrat representing the heavily suburban Guilford District on the Loudoun County Board of Supervisors, stands poised to win the Nov. 3 election.

A few weeks ago, Tatum's Republican challenger, Terry Overstreet, 33, a teacher at Tara-Reston Christian School in Reston, announced that he was dropping out of the race because he and his family, which includes four children, plan to move to a larger home outside of the district.

Richard McCary, local Republican Party chairman, said the Republicans don't plan to field another candidate. The 4.3-square-mile triangular district is bounded by Rtes. 28, 7, and 846 and is northeast of Dulles International Airport.

Opposed or not, Tatum, who was appointed to the board in 1981 to fill a vacancy, said it will not affect her plans to conduct a vigorous campaign. "The voters deserve a campaign," said Tatum. She said the primary issues in Loudoun are growth and transportation.

Tatum has gained influence on the board with a low-key, middle-of-the-road approach to growth and by being well informed and conscientious, according to political observers.

She narrowly won the position in a special 1981 election and was reelected to the seat in 1983. This year, the former owner and operator of the Sterling Pre-School became the first woman to be elected head of the county's governing body.

Tatum has received about $5,000 in contributions and has spent about $3,000 so far.