Former Fairfax County School Board member Janet S. Oleszek was narrowly leading political newcomer Christopher J. Wade as returns came in late Tuesday in the Democratic primary for the Braddock District seat on the Board of Supervisors.

Oleszek had a 42-vote lead over Wade in the race to pick a challenger for the fall contest against Republican incumbent John C. Cook in what has traditionally been a swing district. With all 27 precincts reporting, Oleszek had 50.54 percent of the vote compared with Wade’s 49.45 percent. It was unclear whether there were any provisional ballots to be counted.

In the only other county-level primary, Republican Bill A. Cooper III, 55, of Fairfax Station, held a commanding lead over H. Wes Kammerer, 73, of Centreville, in a duel to determine the challenger to Sheriff Stan G. Barry (D) in November. With results in from 237 of 240 precincts, Cooper had 76.15 percent of the vote compared with Kammerer’s 23.84 percent.

A sliver of Fairfax’s registered voters headed to the polls for the primaries Tuesday. And that was before a 5.8-magnitude earthquake rattled the East Coast, diminishing turnout further, officials said.

Voting continued with paper ballots at some makeshift polling places set up outside buildings that closed after the Virginia-centered earthquake struck shortly before 2 p.m. Election officials said results could be delayed.

Oleszek, 64, entered the race with some name recognition. Having lived in Fairfax for nearly four decades, she won election to the School Board in 2003 and lost a close race against then-state Sen. Ken Cuccinelli II (R) in 2007.

Wade, 38, had the backing of several elected officials, including Board of Supervisors Chairman Sharon Bulova (D). Wade, a father of three, is a businessman and an adjunct instructor at George Mason University. His campaign was dogged by accusations that he was not really a Democrat, owing to his participation in some GOP primaries. But Wade cast himself as an independent.

“I think every vote is going to count in this primary,” Wade said Monday on the eve of the contest. Wade’s wife, Stacy, said he had put everything into the race, all but disappearing for three months while knocking on thousands of doors.

On the eve of the vote, Oleszek expressed confidence in the outcome of the primary and her ability to defeat Cook. She said voters want change, especially to cure Northern Virginia’s traffic problems.

That was true of Thomas Ford, 70, a retired systems analyst who cast his vote at Woodson High School. He said gridlock was the uppermost issue on his mind.

“They need to do something with the roads. It used to be Virginia had nice roads,” Ford said, adding that he would be willing to pay more in gas taxes to improve them.

Ford, a self-described independent who voted for Gov. Robert F. McDonnell (R), also said he had not made up his mind about whom to support in the general election.