Democrats said on Friday they will not swear a North Carolina Republican into his US House seat until state officials resolve questions surrounding his election.

The North Carolina elections board has refused to certify the race between Republican Mark Harris and Democrat Dan McCready, while it investigates irregularities concerning absentee ballots. Harris holds a slim lead in unofficial results, but officials are looking into criminal allegations against an operative hired by his campaign.

Confusing the picture, a state court panel ruled on Thursday that the current elections board should disband at noon on Friday, which it duly did. The state’s Democratic governor, Roy Cooper, announced he would appoint an interim board.

In a statement to the Washington Post late on Friday afternoon, the incoming House majority leader, Steny Hoyer of Maryland, said: “Given the now well-documented election fraud that took place in [the district in question], Democrats would object to any attempt by Harris to be seated on 3 January.”

The US constitution states that the House is the judge of the elections of its members and the final arbiter of all contests.

In North Carolina, Cooper’s move to seek an interim board would allow it to proceed with an 11 January evidentiary hearing about the ninth district race, until a new law governing the statewide elections panel can take effect on 31 January.

Cooper said he would appoint both Democrats and Republicans to comply with pre-2016 state elections law he says is temporarily back in force. But the chairman of the state Republican party, Robin Hayes, argued that Cooper’s proposal was an “illegal sham” and said no Republicans would participate.

Also on Friday, Harris asked the outgoing board to certify him the winner before it dissolved, so he could be seated in Congress. The board turned down his request.

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