Rep.-elect Mark Harris of North Carolina in Washington on Nov. 30. (Michael Reynolds/EPA-EFE/Shutterstock)

NORTH CAROLINA Republican Mark Harris narrowly beat Democrat Dan McCready in the state’s 9th Congressional District. Or so it appeared in the days following last month’s midterm election. Now no one can be sure the vote was honest.

Voter fraud rarely happens in the United States. When it does, it almost always comes in the form of mail-in ballot fraud. Fraudsters can pressure absentee voters to fill out their ballots in a particular way or to hand them over to be filled in by someone else. Unsealed ballots can be changed. Sealed ballots can be trashed, if they come from people who seem likely to vote for the opposition. Play enough of these dirty tricks, and one can change the outcome of a narrow election.

Like, say, the one in North Carolina’s 9th, in which Mr. Harris purportedly beat Mr. McCready by 905 votes out of 283,317 cast. Voters complained that mysterious people were coming to their homes asking to collect absentee ballots. One woman said she signed and then handed over an unsealed and mostly blank ballot. The state received absentee ballots in suspiciously large batches. Before Election Day, state officials had already been concerned about unusual absentee ballot requests, including reports that people were telling voters they needed to re-register and submit absentee ballot paperwork.

Experts examining the count have discovered irregularities. In North Carolina’s Bladen County, Mr. Harris claimed 61 percent of the mail-in vote — though only 19 percent of absentee ballots in the county were requested by registered Republicans. Huge numbers of requested absentee ballots were not returned, suggesting that someone collected and tossed them. Though it is too soon to make any sure conclusions, there was more than enough evidence for the North Carolina State Board of Elections to refuse to certify the district’s vote totals. Investigators are examining Mr. Harris’s slim victory in his primary race, as well.

Yet Republicans are calling on the board of elections to certify the general election count.

Republicans have spent the past decade crying wolf over voter fraud as a pretext for imposing needlessly complicated ID requirements. These laws tend to discourage Democratic-leaning voters. Yet if there is any threat to the integrity of the franchise, it is absentee ballot fraud — which GOP voter-ID laws cannot deter. And the face of this phenomenon in 2018 might well turn out to be a Republican candidate.

States authentically worried about fraud should spend less time hassling voters for IDs and more on monitoring absentee ballots. Elections officials must assess ballot counts for telling irregularities such as those found in North Carolina’s 9th. And they should audit witness signatures on absentee ballot envelopes to determine whether a suspiciously small number of people signed off on a large number of ballots — or whether the names are even real.

Meantime, North Carolina’s elections board should continue to refuse to certify the Harris-McCready race until it is clearer whether there are enough suspicious votes — or non-votes — to have swayed the outcome. The fairest solution may be a new election.