A key U.S. House race in North Carolina where the Republican candidate narrowly leads has now dragged on for a month after Election Day amid concerns about possible election fraud.

The state's Board of Elections declined to certify GOP pastor Mark Harris' unofficial victory over Democratic Marine veteran Dan McCready in North Carolina's 9th District last week as absentee ballot irregularities emerged. Officials in the state now have to assess whether misconduct occurred, as calls grow to refuse to seat Harris in January or even hold a new election.

As tensions mounted in the district, McCready announced during a television interview that he would throw his hat back into the ring.

"As of today, I am withdrawing my concession to Mark Harris," said McCready, who claimed his opponent had been discovered bankrolling "criminal activity."

"Furthermore, I call on Mark Harris to tell the American people exactly what he knew and when he knew it."

McCready tweet

The Harris campaign did not immediately respond to CNBC's request for comment on McCready's announcement. The McCready campaign also did not respond to a request for comment.

The elections board has to hold an evidentiary hearing by Dec. 21, and can order a new election if it determines that irregularities marred the election's result or cast doubts on its fairness. Harris currently holds a 905-vote lead, and it is unclear whether the possible fraud went far enough to swing the race in the Republican's favor.

If the result changes, it would flip another GOP-held seat to Democratic control. Democrats carried at least 40 Republican districts in last month's midterm elections on their way to winning a House majority.

The votes in question largely stem from Bladen County, where Harris won 61 percent of the mail-in vote despite Republicans requesting only 19 percent of absentee ballots in the county, according to The Washington Post. Bladen had an absentee ballot request rate of 7.5 percent of registered voters, compared with about 3 percent in most counties, The New York Times reported.

Reports have also pointed to irregularities in this year's Republican primary, when Harris narrowly beat Rep. Robert Pittenger. The representative privately blamed the "ballot stuffers in Bladen" after he lost in May, the Post reported Thursday. Pittenger's campaign warned the executive director of the state GOP and a regional director for House Republicans' campaign arm, but officials "did little to scrutinize the results," according to the newspaper.

At the center of the possible irregularities sits McCrae Dowless, an electioneer and elected vice chairman of the Bladen Soil and Water Conservation. Two women told Charlotte TV station WSOC that Dowless paid them to pick up voters' absentee ballots.

The outlet also cites affidavits in which voters claim a person picked up their absentee ballots. In one case, a voter said her ballot was uncompleted when it was taken.

It is unclear how closely linked Dowless is to the Harris campaign, or exactly what effect his alleged conduct had on the race. CNBC could not immediately reach Dowless through the Bladen Soil and Water Conservation. The Charlotte Observer reported that Dowless had denied any wrongdoing.