The number of registered voters in Virginia increased by only 2 percent in the first nine months of this year, a sluggish pace that politicians say indicates little interest in a state lottery referendum on the Nov. 3 ballot.

But Susan H. Fitz-Hugh, Virginia's top voter registration official, says that hotly contested local elections pushed registration significantly higher in scattered pockets of the state, including Fairfax and Prince William counties.

In Fairfax, where Board of Supervisors Chairman John F. Herrity, a Republican, is in a tough fight with Democratic Supervisor Audrey Moore, registration is up 4.4 percent. The county's total number of voters jumped by 16,162 since Jan. 1, to 385,991.

And in Prince William County, Democratic Commonwealth's Attorney Paul B. Ebert is waging a big-budget battle against Republican challenger Peter W. Steketee. Registration there is up 4.7 percent, an increase of 2,890 voters to a total of 64,383.

Fitz-Hugh, executive secretary of the State Board of Elections, said yesterday that overall, Virginia picked up slightly fewer than 50,000 voters from Jan. 1 to Oct. 3, raising the total electorate to 2,659,095.

Oct. 3 was the last day people could register to vote in the Nov. 3 elections, in which Virginians will select all 140 members of the General Assembly and cast ballots in an advisory referendum on whether to establish a state lottery.

"A lot of people thought the lottery would increase voter registration significantly this year. These figures don't support that," Fitz-Hugh said. "The net increase in voters this year is lower than the increase in 1983," the last election in which all seats in the legislature were at stake.

"The question five months ago was, 'How is the lottery going to affect the election?' " said Virginia Democratic Chairman Lawrence E. Framme III. "It doesn't seem to have had any significant effect at all. It's the quietest lion I've ever seen."

Earlier this year, when lawmakers put the lottery question on the ballot, many political activists predicted it would dominate the political season. But neither the lottery's advocates nor its detractors have mounted a highly visible campaign, and analysts now say voter turnout is likely to determine whether the referendum passes.

Polls have consistently shown that about 60 percent of the state's voters support a lottery, with 40 percent opposed or undecided. But analysts say a small turnout would favor opponents, who include well-organized church groups that consider a lottery immoral.

Although voter rolls have increased from last year, the total is still below the state's all-time high in 1984, the last presidential election year, when registration was slightly less than 2.7 million. Traditionally, registration peaks in presidential election years and drops sharply the next year when rolls are purged of those who have not voted in four years.

Voter registration totals in other Northern Virginia localities include:Arlington County: 88,745, an increase of 880.

Alexandria: 54,940, an increase of 818.

Loudoun County: 33,290, an increase of 875.

Fairfax City: 11,636, an increase of 90.

Manassas: 7,757, an increase of 422.

Falls Church: 5,382 a decrease of 51.