With no strong opposition expected in the races for U.S. Senate and House of Representatives on the November ballot in Loudoun County, the Board of Supervisors' Sterling District has become a focal point of electoral interest.

Democrat Howard P. Smith, appointed to the Board of Supervisors last fall as the Sterling District representative, and Republican nominee H. Roger Zurn Jr. already have staked out positions for the campaign. At the same time, both men and other people active in county politics are also thinking about 1991, when the entire Board of Supervisors will be running for four-year terms.

Several Loudoun Republicans have said that a loss by Smith could mean that voters are dissatisfied with the Democratic-controlled Board of Supervisors and that voters would therefore be inclined to vote in a Republican majority next year.

"There's a lot of interest in terms of how this election will turn out," Zurn said.

Smith and other Democrats agree that the Sterling District campaign is an important one, but they say it would be wrong to interpret the November results as a referendum on the current board, no matter who wins. Issues and candidate qualifications, rather than possible frustration with the entire Board of Supervisors, will tip the scales this fall, Democrats say.

No Democrat has been elected to the Sterling District seat for a decade, officials of both parties say.

Last October, after the resignation of independent Alice Bird from the Sterling District seat, the five Democrats and two Republicans on the county board selected Smith to join them until the special election is held in November. The supervisors picked Smith over Zurn, two other Republicans and one other Democrat for the $18,000-a-year position.

Yet Democrats and Republicans alike say incumbency is not likely to be a major factor this fall. Leadership likely will be, however.

"I have represented the people of Sterling as well as I could have expected to," said Smith, a retired cartographer who favors establishing parks and preserving open space. Although he first became active in local civic affairs a little more than one year ago, he says he's proud of his record and what he described as the course set by the county board as a whole: "to go forward with measured growth, and to push for affordable housing soon."

Not surprisingly, Zurn sees things differently. Smith "hasn't bothered to stay in touch with the people of his district," said the Republican, who owns Loudoun Temporary Services in Countryside. "He's basically done everything by the seat of his pants."

Zurn, a member and former chairman of the county's Affordable Housing Advisory Committee, said young people in Sterling "have absolutely nowhere to go," leading to boredom and fights. He advocates establishing a teen center and creating more playing fields.

He said the board as a whole has "short-changed" the Sterling District and has failed to provide a clear direction to its staff on countywide policy. "Now the staff leads the board," Zurn said, noting that the supervisors have delayed action or deadlocked on some key issues, such as a new county government complex.

Responded Smith, "Leadership is always in doubt when you have a board." The Democrat says nearly three decades of conflict between the developed east and undeveloped west of Loudoun may be fading.

Both men say transportation is important, but they have shown few policy differences on the subject so far. Each says a western bypass that might go through Loudoun must be considered if it can ease local traffic. In addition, each candidate describes himself as a fiscal conservative, each is willing to debate the other, and neither plans to spend more than $10,000 on the campaign.

"I don't think the election will be very close," said Loudoun Democratic Committee Chairman Al Sitterson in predicting a Smith victory this fall.

Sitterson's GOP counterpart, William Mims, says Zurn's "mainstream Republican views" may give him the edge.