HELENA, Mont. (AP) — An investigation into whether political operatives in North Carolina illegally collected and possibly stole absentee ballots in a still-undecided congressional race has drawn attention to a widespread but little-known political tool called ballot harvesting.

It’s a practice long used by special-interest groups and both major political parties that is viewed either as a voter service that boosts turnout or a nefarious activity that subjects voters to intimidation and makes elections vulnerable to fraud.

The groups rely on data showing which voters requested absentee ballots but have not turned them in. They then go door-to-door and offer to collect and turn in those ballots for the voters — often dozens or hundreds at a time. Some place ballot-collection boxes in high-concentration voter areas, such as college campuses, and take the ballots to election offices when the boxes are full.

In North Carolina, election officials are investigating whether Republican political operatives in parts of the 9th Congressional District harvested ballots from minority voters and didn’t deliver them to the election offices. In some cases they are accused of harvesting ballots that were not sealed and only partially filled out. Ballot harvesting is illegal under state law, which allows only a family member or legal guardian to drop off absentee ballots for a voter.

Investigators are focusing on areas in the district where an unusually high number of absentee ballots were not returned. They want to know whether some ballots were not turned in as promised to the local elections office, were unsealed or only partially filled out.

Republican Mark Harris leads Democrat Dan McCready by 905 votes, but the state elections board has refused to certify the results. The head of the state Republican Party said Thursday that he would be open to holding a new election if there is evidence of fraud.

Supporters of ballot harvesting say they worry the North Carolina election may give an important campaign tool an unnecessary black eye. These groups see their mission as helping voters who are busy with work or caring for children, and empowering those who are sick, elderly and poor. Collecting ballots to turn in at a centralized voting hub also has been an important tool for decades on expansive and remote Native American reservations.

“Sometimes we think of voting as this really straightforward process and we often forget that all voters, but for new voters in particular, there’s a lot of confusion when voting about when they actually have to vote by, where they have to take their ballot to,” said Rachel Huff-Doria, executive director of the voter advocacy group Forward Montana.

Several states have tried to limit ballot harvesting by restricting who can turn in another person’s ballot. In Arizona, a video that showed a volunteer dropping off hundreds of ballots at a polling place prompted a debate that led to an anti-ballot harvesting law in 2016.

“I think at any level, Republican, Democrat or anything, it’s wrong. It’s a terrible practice,” said former Arizona Republican Party chairman Robert Graham, who backed the law. “People should be responsible for their own votes.”

The Arizona law making it a felony in most cases to collect an early ballot was challenged in federal court before the 2016 election, and blocked by an appeals court. The U.S. Supreme Court stepped in and allowed the law to be enforced.

Further challenges have so far been unsuccessful, most recently just before the midterm election.

Montana was the latest state to pass an anti-ballot harvesting law when voters approved a referendum last month. Al Olszewski, a Republican state senator, said he proposed the ban after two of his constituents in northwestern Montana complained of pushy ballot collectors coming to their homes.

“For a woman in her 70s that’s maybe frail and lives alone and feels intimidated, at least now they can say please leave” and have confidence that the law is behind them, he said.

Voting-rights advocates are dismayed that such laws are being passed without evidence of actual ballot fraud happening, at least before questions were raised about the activities in the North Carolina congressional race. They say restricting who can collect ballots punishes certain voters without doing anything to actually detect, deter or punish fraud.

“If you have an honest person who is trying to help voters, then who they are doesn’t matter as long as they return (the ballot),” said Myrna Perez, the deputy director of the Brennan Center for Justice’s democracy program.

California went in the opposite direction when it passed a law in 2016 to allow ballot harvesting.

Republicans felt the new law’s effects during this year’s midterm elections after congressional districts that GOP candidates were leading on Election Day flipped to the Democrats when a flood of last-minute mail-in ballots were counted along with provisional ballots.

The rout included several seats that had been held by Republicans in the former GOP stronghold of Orange County, where more than 250,000 mail-in ballots were turned in on Election Day. And in the agriculture-dominant Central Valley, Republican incumbents Jeff Denham and David Valadao saw their leads disappear after a tally of late-arriving ballots.

Valadao, for example, had an initial lead of more than 7 percentage points, but Democrat T.J. Cox pulled ahead after winning 56 percent of the votes counted after Election Day.

Republican House Speaker Paul Ryan described California’s election system as “bizarre” in an interview with The Washington Post.

California’s situation underscored that ballot harvesting is an important tool for political parties. Orange County Republican Party Chairman Fred Whitaker wrote in a newsletter last month that Republicans must “develop a response to this new law that allows us to remain competitive.”

Even Olszewski, the sponsor of Montana’s anti-ballot harvesting measure, acknowledges that laws such as his are unlikely to eliminate ballot harvesting completely. Such “micro-targeting” of voters when used with technology to identify individuals’ political leanings has become too important and effective in get-out-the-vote efforts, he said.

“I think the Democrats, they’re the ones that figured it out and were far more successful in ’18, in this election, than the Republicans ever were,” he said. “The Republicans, what I’m hearing right now early on is, holy cow, we need to learn how to do this as good or better than the Democrats at harvesting ballots. We have the data.”

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Associated Press writers Michael R. Blood in Los Angeles and Bob Christie in Phoenix contributed to this report.