The Fairfax County Electoral Board is investigating a possible irregularity in the number of absentee ballots cast in Virginia’s largest jurisdiction that Democrats say could shift votes in the still-unresolved race for Virginia attorney general.

As of Thursday evening, state Sen. Mark D. Obenshain (R-Harrisonburg) led state Sen. Mark R. Herring (D-Loudoun) in the contest by 777 votes – or .03 percent of the 2.2 million votes cast — according to the State Board of Elections’ Web site. Local election boards are now counting provisional ballots, cast by people without ID or in the wrong polling place, and canvassing the returns looking for any possible errors. Both campaigns have said they will consider asking for a recount, depending on the results of the review.

One oddity was flagged in Fairfax County by the political team of Rep. Gerald E. Connolly (D-Va.).

The State Board of Election’s site shows absentee ballots cast in each county broken down by congressional district. Fairfax County includes portions of three districts: Connolly’s 11th, Rep, Frank R. Wolf’s (R) 10th and Rep. James P. Moran Jr.’s (D) 8th.

According to state numbers, Fairfax reported an unexplainably lower number of absentee ballots cast in the 8th District than in the other two congressional districts.

The county keeps track of how many voters request absentee ballots as well as the number who actually turn them in. In the 10th District, 88 percent of voters who requested a ballot actually voted, while 86 percent did so in the 11th District.

But in the 8th District, the state board shows that only 50 percent of those who requested ballots — 4,168 out of 8,363 requests — actually cast ballots, a response rate not only lower than the other portions of Fairfax County, but lower than any other congressional district in the state, according to the Connolly campaign.

Seth Stark, the chairman of the Fairfax Electoral Board, said in a brief interview that he “just became aware” of the matter Thursday night.

“We’re going to take a look at it tomorrow,” Stark said Thursday.

A Republican member of the electoral board, Brian Schoeneman, agreed in several tweets late Thursday that a discrepancy exists. “I am convinced now too that there is an issue,” he tweeted.

“Top priority tomorrow will be canvass of 8th District [absentee ballots],” he said Thursday. “We will figure this out.”

The 8th District portions of Fairfax are more heavily Democratic than the rest of the county, with more than 70 percent of voters in many of its precincts having voted for Herring in Tuesday’s contest. In other words, any discovery of previously uncounted absentee votes in the 8th District is likely to benefit Herring more than Obenshain.

Another way of looking at the discrepancy was in the total number of absentee ballots cast. According to a news story published by WUSA-9 before the election, Fairfax County reported on Nov. 2 that more than 24,300 voters had cast absentee ballots. Yet according to the state Board of Elections’ Web site Thursday, the county reported only 22,484 absentee votes cast in the attorney general’s race.

It’s unclear why those two numbers are different, or whether a recording error occurred when Fairfax reported its numbers or when the state board recorded them. The discrepancy in those two numbers was first pointed out on Twitter late Thursday by Dave Wasserman of the Cook Political Report.

The cause of the irregularity was unknown as of late Thursday.

Kevin O’Holleran, a spokesman for Herring’s campaign, said the Democrat was also eager to get to the bottom of the issue.

“We feel very strongly that Mark Herring will be the next attorney general,” O’Holleran said. “Based on a number of significant indicators, there may be thousands of unaccounted absentee votes in Fairfax County. This issue needs to be addressed immediately to ensure every Virginian’s vote is counted.”

Obenshain spokesman Paul Logan said: “The race is extremely close. We are confident that we will prevail. We’re going to wait until the State Board of Elections finishes its tabulations and make sure that every legitimate vote is counted.”