A former Loudoun County supervisor, Bob Scheetz, who recently switched to the Republican Party, is challenging a Democratic incumbent in a special election Nov. 3 for a seat on the heavily Democratic Board of Supervisors.

Democrats, who now have a 6-to-2 board majority, appointed Betty W. Tatum, 40, to the board seat last July to fill a vacancy left by the resignation of former supervisor Gerry Gardner, the board's first woman member. Tatum is seeking to hold off a challenge by Scheetz, 46, in the Guilford district at the county's more developed and urban-oriented eastern edge.

The contest is taking place amid GOP efforts to make further inroads in traditionally Democratic Loudoun in the fall elections. The winner will serve the remainder of Gardner's term through 1983. Gardner resigned in June, citing personal reasons, and has left Virginia.

Scheetz, a Sterling Park real estate broker, was appointed to the newly created Guilford district seat as a Democrat in 1975 and served 14 months. He was defeated by Gardner in the 1976 Democratic primary. He says he switched his party affiliation this year.

Tatum was named to the Loudoun planning commission last year and has been Guilford Democratic Party chairman since 1972. She runs a half-day preschool center at her home and teaches piano.

In his campaign, Scheetz has focused on a controversial proposal for a Sheraton hotel and conference center on Rte. 7 in Loudoun. Scheetz favors county approval of the proposal, saying the complex is expected to provide new tax revenue and jobs for Loudoun. Tatum, he contends, is "not sufficiently aware of what economic development means."

Tatum voted against the Sheraton proposal as a planning commission member but now says she has an "open mind" about whether the Board of Supervisors should approve a modified version of the plan. She argues that the proposal should be further altered to shift an exit road from state Rte. 7 to Rte. 641. The entire Sheraton plan has stirred considerable controversy among Loudoun residents, planners and preservationists.

Scheetz appears to be outspending Tatum in the campaign. Scheetz says he has raised about $2,500 so far. Tatum says she has collected nearly $1,000.

In her campaign, Tatum urges a closer relationship between the Board of Supervisors and the school board and she argues that industrial development should be promoted largely in commercial sections like the Dulles International Airport area.

Scheetz says he favors economic development to help increase the county's tax revenues.