Loudoun County officials say that unexpectedly large number of supporters of political extremist Lyndon H. LaRouche Jr. have registered to vote there, raising fears that the new voters will stage write-in campaigns for LaRouche backers in next month's municipal elections.

More than 110 persons went to the county registrar's office in Leesburg earlier this month in the two days before the deadline to vote in the May 6 elections for local offices in the towns that dot the Loudoun countryside.

County officials said they were surprised by the sudden display of voter interest and have identified 80 or more of the new registrants as LaRouche supporters.

In all, officials who have examined the county voter registration lists say there may be 200 LaRouche supporters registered for the local elections.

While the number may appear small, county officials note that the turnout in municipal elections historically is low and that two years ago as few as 13 votes could have elected a Town Council member in Hillsboro in western Loudoun. In Leesburg, the county's largest town, a margin of 63 votes separated the winning and losing candidates for Town Council in the last election.

"I think there is no question the LaRouche people will vote as a unit," said Loudoun Supervisor Frank Raflo of Leesburg, an outspoken LaRouche opponent. "In small communities, five or ten people can make a difference."

"It obviously means that the LaRouche people hope to have an impact on the elections," said Leesburg Mayor Robert E. Sevila, who is unopposed for reelection. " . . . You have to be mindful that they may be out there conducting an invisible, behind-the-scenes campaign."

Sevila said he concluded that most of the new people on the voter rolls are LaRouche supporters because their names have appeared as bylines in the Loudoun County News, a LaRouche-affiliated newspaper, and many of the people listed previous addresses in New York City.

Dana Scanlon, a spokeswoman for the LaRouche organization, declined to comment when asked of the group's political intentions in Loudoun.

The LaRouche-affiliated newspaper has criticized county officials in almost every issue, making many wary of the LaRouche supporters and their intentions.

"What impact could they have if they decided they wanted to have an impact?" asked Mayor Sevila. Most of the LaRouche backers, Sevila and other officials said, live in Leesburg, with a few in Middleburg, Hamilton, Lovettsville and other towns in western Loudoun.

A perennial presidential candidate, LaRouche moved to the county in 1983 and last year began moving his political operations, along with several hundred employes, there from Manhattan.

LaRouche, who lives in a $1.3 million estate protected by armed guards near Leesburg, recently alarmed national Democratic Party officials when two of his supporters won upset victories for the party's nominations for Illinois lieutenant governor and secretary of state.

Democratic leaders in Delaware will hold seminars to help their members identify candidates allied with LaRouche, a party official said yesterday. The LaRouche organization has said it has supporters running in 2,000 races nationwide.

Loudoun officials said that opposition to LaRouche runs high in the county, making a victory by any candidate formally backed by the LaRouche organization unlikely. But they say that the political climate in many towns could help any LaRouche movement.

Town races often go uncontested, and there are often more seats on the Town Council than there are announced candidates. In Hillsboro, Virginia's smallest incorporated town, there is no announced mayoral candidate, and only one candidate for five seats on the Town Council. In 1984, a write-in candidate was elected with 13 votes.

Loudoun officials see serious implications if the LaRouche organization is successful in placing one of its supporters in office in Loudoun .

"I'm sure that they would point to that both nationally and locally as evidence of their acceptance in our community," Sevila said.