Law enforcement officials are now examining Mr. Dowless’s operation and could ultimately bring charges against him and his associates. But the five-member elections board is also weighing whether Mr. Dowless’s behavior amounted to “irregularities or improprieties” that were so damaging that they tainted the entire election.

The board’s decision, which could come this week, might prompt a new election and is certain to reverberate through the Ninth District and into Congress.

[Read: In North Carolina, Investigators Find Ballot ‘Scheme’ in House Race]

Mr. Dowless, 63, who has a criminal record that includes felony convictions for perjury and fraud and has worked for politicians in both parties, has declined repeated requests for interviews, and, through his lawyer, refused on Monday to testify before the state board. The panel’s chairman warned that the board could still “take negative inferences concerning his actions.”

On Wednesday, one of Mr. Harris’s sons, John Harris, testified that he had warned his father in April 2017 about his misgivings about Mr. Dowless, whose work had been scrutinized by the state just months earlier. John Harris said he had become suspicious after analyzing returns from a 2016 campaign that his father had lost — and that included a candidate who had hired Mr. Dowless.

“The key thing that I am fairly certain they do that is illegal is that they collected the completed absentee ballots and mail them all at once,” John Harris wrote in an email to his parents on April 7, 2017, one day after Mark Harris, a longtime pastor then considering a 2018 run, met with Mr. Dowless and was told that his “absentee ballot program” was fully legal.

“They believed what McCrae assured them,” John Harris said of his parents on Wednesday. “I didn’t.”

The elder Mr. Harris, who sometimes wiped away tears as his son testified, and who is expected to take the stand himself on Thursday, hired Mr. Dowless anyway. During the closely fought primary and general elections that followed, tens of thousands of dollars in fees and reimbursements passed from Mr. Harris’s campaign to a consultant to Mr. Dowless.