The incoming House majority leader said Democrats might refuse to seat a North Carolina Republican next year unless and until “substantial” questions about the integrity of his election are resolved.

Rep. Steny H. Hoyer (D-Md.), the current minority whip, made the comments to reporters Tuesday as North Carolina election officials investigate whether an operative working on behalf of Republican candidate Mark Harris illegally collected incomplete ballots from voters.

Hoyer’s comments, and the increasing criticism from other national Democrats, represent a new threat to Harris’s candidacy — suggesting that even if his apparent narrow victory is ultimately certified by the state, Harris could be subject to a months-long process in the House to determine whether he is ultimately sworn in.

The state probe has delayed the certification of Harris’s race in North Carolina’s 9th District, and state officials could decide to call for a new election. Harris and Democrat Dan McCready are separated by 905 votes, according to unofficial returns.

“If there is what appears to be a very substantial question on the integrity of the election, clearly we would oppose Mr. Harris being seated until that is resolved,” Hoyer said.

Meanwhile, key Republicans have gone silent or dialed down their rhetoric as the scope of the allegations have emerged in recent days.

Dallas Woodhouse, executive director of the state GOP, charged in a fundraising email and a robo-call Monday that the allegations were a warm-up for Democratic plans to steal the 2020 presidential election.

On Tuesday, he offered no fire and brimstone, saying that the investigation should be thorough while also suggesting a new election would not be fair to the people who voted this year.

“I think in no way do we want to be seen supportive of anything untoward,” he said, adding that it would be a “real perilous situation” if the election were abrogated.

Democrats, meanwhile, sought to draw national attention to the North Carolina allegations.

Rep. Hakeem Jeffries (D-N.Y.), chairman of the House Democratic Caucus, tweeted Tuesday that it “appears increasingly likely that North Carolina Republican operatives stole a House seat,” while Rep. Seth Moulton (D-Mass.), whose Serve America PAC supported McCready, called for a fresh election.

“Crickets from the Trump Administration,” Jeffries wrote, referencing President Trump’s unfounded claims of rampant voter fraud in the 2016 presidential election. “So here’s a thought: LOCK THEM UP.”

Rep. Cheri Bustos (D-Ill.), chairwoman of the Democratic Congressional Campaign Committee, said in a statement that there is “extensive evidence of impropriety and potential election fraud” but stopped short of calling for a new election.

“The bottom line is this, if there proves to be fraud, then there must be consequences,” she said. “The voters of North Carolina’s 9th Congressional District should expect nothing less.”

National Democrats are keeping a close eye on the allegations, which are largely concentrated in two North Carolina counties where Republican operative Leslie McCrae Dowless allegedly oversaw a crew of workers who collected absentee ballots from voters. Some of those ballots, according to affidavits filed with state authorities, were incomplete when they were gathered.

Top Republican officials in Washington have kept quiet about the allegations as the state investigation is underway. A spokeswoman for Rep. Kevin McCarthy (R-Calif.), who will be the top House GOP leader next year, did not respond to a request for comment on Hoyer’s suggestion that Harris might not be seated, nor did a spokesman for the National Republican Congressional Committee.

Michael Steel, a Republican strategist and North Carolina native, said it would be prudent for GOP leaders in Washington to wait and see what the state probe reveals.

“If the state certified Harris as the winner and the Democratic majority objects to seating him, that is something national Republicans should object to,” he said. “But for the time being, this is a state issue, and we should respect the process being carried out by the state of North Carolina.”

Another prominent Republican with North Carolina roots, former Republican National Committee spokesman Doug Heye, tweeted Tuesday: “What is happening in North Carolina is outrageous and an embarrassment.”

National Democrats, meanwhile, are moving aggressively to investigate and publicize the fraud allegations.

The DCCC has lawyers and staffers combing Bladen and Robeson counties in North Carolina in concert with state party officials to track down and interview voters and campaign workers who interacted with Dowless, according to a Democrat familiar with the effort but not authorized to discuss it publicly.

“It’s an all-hands-on-deck situation, and our intention is to leave no stone unturned,” the Democrat said.

The North Carolina State Board of Elections has indicated it will hold a hearing on the allegations by Dec. 21 and could decide afterward whether to certify the election, call a new election or take some other course of action.

If Harris is certified as the winner, the matter would rest solely with the House, which has the express constitutional authority to judge the “elections, returns and qualifications” of its members.

Hoyer said he intended to discuss the matter with Rep. Zoe Lofgren (D-Calif.), who is expected to chair the Committee on House Administration next year. Under federal law, that panel is empowered to independently investigate the election and make recommendations about how the dispute should be resolved — including calling for a new election.

Hoyer made reference to the contested 1996 election between then-Rep. Bob Dornan (R-Calif.) and Democratic challenger Loretta Sanchez. Sanchez won by fewer than 984 votes, but Dornan later claimed that the election had been stolen, because of illegal votes from non-U.S. citizens.

A task force appointed by what was then called the Committee on House Oversight found evidence of 748 tainted votes, not enough to change the outcome of the election, and dismissed Dornan’s challenge.

In 1985, an Indiana congressional seat remained vacant for five months while a panel investigated whether to seat the Democrat or the Republican after a disputed election. The matter ended in partisan rancor after the Democratic majority voted along party lines to seat the Democrat, Rep. Frank McCloskey.

“The House has, as you know, the authority over the propriety of the election,” Hoyer said. “This is a very substantial question; it ought to be resolved before we seat any member. . . . I would hope the North Carolina officials get to the bottom of this controversy.”

Amy Gardner and Kirk Ross in Raleigh, N.C., contributed to this report.