Investigators on Monday will begin revealing the breadth of an alleged ballot-tampering scheme in North Carolina, launching a potentially bitter and partisan battle over how to fill a congressional seat that has remained vacant since January.

When it opens a long-anticipated hearing Monday morning in Raleigh, the North Carolina State Board of Elections will be listening for evidence to judge whether enough ballots were affected in tiny Bladen County to taint the outcome in the 9th Congressional District. The board has the power to call for a new election or certify the November results.

The election has been in limbo since November, when evidence surfaced that a political operative paid by Republican candidate Mark Harris had assembled a crew of election workers to collect mail-in ballots from voters, a felony in North Carolina. With Harris leading Democrat Dan McCready by just 905 votes, the board declined to certify the results and launched a wide-reaching investigation instead.

The allegations have prompted Democrats to demand a new election, while Republicans, citing the absence of public evidence that fraud affected the outcome, have called repeatedly for Harris to be sworn in.

The investigation has refocused the national debate about election fraud. Republicans, led by President Trump, have alleged widespread voter fraud and advocated strict ID laws and criminal prosecutions. Democrats have argued that the kind of in-person fraud Republicans have targeted is rare, accusing their opponents of trying to hinder ballot access and intimidate voters who typically vote Democratic. The North Carolina case reverses those roles in some ways.

Adding to the partisan animosity, the state elections board requires a supermajority of four votes to call for a new election. With three Democrats and two Republicans, the board will not have the votes to take any action at all if its members vote along partisan lines. That would turn attention to Congress, which also has the power to order a new election.

Neither side knows what the state’s evidence will show. Two big unanswered questions: Did Harris, a 52-year-old evangelical pastor from the suburbs of Charlotte, know about the alleged scheme? And were enough ballots affected by fraud to change the outcome of the election?

No evidence has emerged so far to show either, although Harris personally directed the hiring of the aide, an adept field operative and Bladen County native named Leslie McCrae Dowless, despite being warned about his tactics. Harris has said repeatedly that he had no knowledge of Dowless’s allegedly illegal tactics.

“We don’t know what the allegations are,” said Harris lawyer David Freedman. “We don’t really know who the witnesses are going to be. We’ve not seen any sort of report. I can’t think of any proceeding I’ve ever gone into with less information in terms of what evidence the state board investigators have. Which makes it difficult to be prepared to defend, because we don’t know what we’re defending.”

Witnesses in interviews and affidavits said Dowless’s allegedly fraudulent operation on Harris’s behalf involved collecting incomplete and unsealed ballots, an illegal practice. Investigators are also examining whether Dowless or those working for him illegally discarded ballots.

Dowless, 63, was investigated in 2016, when he helped deliver an overwhelming share of the mail-in vote in Bladen County for a different Republican congressional candidate, Todd Johnson. The 9th District stretches along the South Carolina border from Charlotte to rural, eastern North Carolina.

Former Rep. Robert Pittenger, whom Harris defeated in the Republican primary last June, told The Post in December that Dowless approached him in 2016 but he declined to hire him. “I just knew I didn’t want to be involved with him,” Pittenger said. “Dowless’s efforts were widely known, and we did share our concerns with several people.”

Since opening the most recent probe late last year, investigators have published reams of evidence and affidavits on the state board’s website, including examples of absentee ballots allegedly collected and turned in by Dowless or his crew.

The evidence has not shown that enough ballots were tampered with to change the outcome of the election, which has prompted the Harris campaign to assume that such evidence doesn’t exist.

Investigators are likely to address whether anyone inside the Bladen County Board of Elections was aware of the scheme and participated in it — a prospect that would allow the board to declare the results generally tainted, which state law allows as grounds for calling for a new election.

The board published evidence in December that the county election staff improperly released early-voting results, but it is unknown if the results were given to Dowless.

The lack of information has not stopped the parties from drawing their battle lines ahead of the hearing.

In a brief filed this week with the state board, the McCready campaign, represented by prominent Democratic lawyer Marc Elias, argued that the alleged ballot tampering taints the entire election and warrants calling for a new one.

“Evidence of ballot tampering performed by Harris’s handpicked operative casts a cloud of suspicion on all the [absentee] ballots cast in the election,” the McCready brief states, and is “more than sufficient” to justify calling for a new election.

Harris’s team, in turn, will argue for certification, Freedman said.

“With as many investigators as have been down there, reporters and our own investigation, it’s hard for me to believe there are matters that will arise that have not already been exposed,” Freedman said.

Still, there was evidence of caution on both sides. The GOP has claimed widespread fraud to justify strict new voter identification laws and far-reaching crackdowns on noncitizen voting; some in the party have noted the potential contradiction of downplaying evidence of the alleged election-fraud scheme in Bladen.

And some Democrats in Washington said they are grappling with the risk of overreaching if they call for a new election without overwhelming evidence that ballot-tampering was widespread enough to affect the outcome in November.

McCready, 35, argued in his brief that if the state board fails to call for a new election, the U.S. House should then do so. But not all Democrats on the Hill relish that opportunity.

“House leaders are cognizant of how far they should push to set new precedent here,” said a senior Democratic staffer on Capitol Hill who was not authorized to speak publicly. “But they also recognize that this is a potentially unprecedented situation, if the state board can neither certify the election nor order a new one.”

Anticipation for Monday’s hearing is so high that officials reserved a 120-person courtroom for three days.

In some ways, the hearing will have the look and feel of a trial, with board investigators and the two candidates all planning to call their own witnesses and cross-examine those called by others.

Perhaps the star witness could be Dowless himself, though it’s unknown if he will agree to testify or decline to do so to avoid self-incrimination, as he did at a hearing over similar irregularities more than two years ago.

Also on the witness lists, among dozens of others: Harris; McCready; North Carolina GOP Executive Director Dallas Woodhouse; and Andy Yates, the GOP consultant who worked for Harris and hired Dowless at Harris’s direction.

Neither Dowless nor his lawyer, Cynthia Singletary, responded to requests for comment. A lawyer for Yates, Mark Jones, said Yates will testify if he receives a subpoena to do so. North Carolina law allows the state board to compel witnesses to testify but requires it to grant immunity when it does.