[A state official shared some of the election board’s conclusions on Monday.]

Although Republicans have held the Ninth District’s House seat for more than a half century, it was in peril last fall, when Democrats were on the brink of making substantial gains in Congress and the party had fielded a candidate they felt was among the strongest in memory in the Ninth. But the election board, which has the power to decide whether to order a new election, found that a Republican operative oversaw an elaborately concealed get-out-the-vote effort they came to believe was endemic with fraud and misconduct to benefit Mr. Harris.

According to Ms. Strach and the testimony that followed on Monday, L. McCrae Dowless Jr., the political contractor for the Harris campaign, orchestrated the effort. It included the handwritten completion of some ballots and, in violation of North Carolina law, the in-person collection of absentee ballots. In Bladen County, where Mr. Dowless primarily worked, Mr. Harris won 61 percent of the received absentee ballots, even though registered Republicans accounted for only 19 percent of the ballots submitted.

According to Mr. Yates, who did not invoke his constitutional protection against self-incrimination or testify with immunity, Mr. Dowless repeatedly and explicitly pledged that his work was within the confines of state law, which generally forbids third parties from handling absentee ballots.

“Mr. Dowless told me that he knew it was illegal to collect ballots and that he told all his workers that it was illegal to collect ballots, that they would never collect ballots, that he never touched or handled a ballot,” Mr. Yates said. “I was shocked and disturbed to learn that that appears that it was not the case. If that had ever become evident to me in the campaign, I would have immediately cut off all contact.”

Still, Mr. Yates’s testimony risked feeding perceptions that the Harris campaign was negligent, by design or happenstance, in its dealings with Mr. Dowless, who had convictions for perjury and insurance fraud and had been the subject of scrutiny — but not prosecution — for his political work. The testimony, though, was a lengthy account of some of the Harris campaign’s interactions with Mr. Dowless, whose hiring was personally directed by the candidate.