By Mr. Harris’s own account on the stand Thursday, it was his narrow defeat in the 2016 race that set a course toward the board’s decision. By March 2017, according to a text message turned over to investigators only on Wednesday, Mr. Harris was communicating with a political ally in Bladen County, a rural part of the district, about “the guy whose absentee ballot project for Johnson could have put me in the US House this term, had I known, and he had been helping us.”

“Johnson” was Todd Johnson, one of Mr. Harris’s rivals in the 2016 race, and “the guy” was Mr. Dowless, a Bladen County operative with a felony record and a reputation for shadowy work for Democratic and Republican politicians. Mr. Dowless and Mr. Harris met in a furniture showroom about a month later.

“How did you beat us so bad?” Mr. Harris said he had asked Mr. Dowless about the 2016 race. Mr. Dowless then explained a two-phase effort that concentrated on absentee ballots.

In the first phase, Mr. Harris testified, Mr. Dowless and his associates would help voters submit requests for absentee ballots — an ordinary and legal political activity in North Carolina. In the second, workers would ensure that voters had received the ballots and would inquire whether they needed assistance.

But under no circumstances, Mr. Harris was assured, were the workers to collect the ballots.

Two weeks after the meeting, Mr. Harris pulled out his personal checkbook and paid a $450 retainer. The next month, before Mr. Harris entered the congressional race, he paid Mr. Dowless another $2,890.

Mr. Dowless and his associates, who were often his acquaintances or relatives with little interest in politics but hoping for fast cash, went to work. Mr. Dowless, who refused to testify before the board, has not been charged with any crimes in connection with the 2018 election, nor have any of his workers. Prosecutors are examining the operation, though, and are considering whether to bring any criminal charges.