More than 900,000 Georgians have cast mail-in ballots so far in the Jan. 5 Senate runoff election, but a small number, some 3,400, are getting an outsized amount of attention in the closing days of the race.

This small sliver of ballots have been rejected by local election offices because of problems with signatures or other issues that require correction if they are to be counted. With control of the U.S. Senate hanging in the balance, Democrats have intensified their efforts to cure the rejected ballots and get them counted by the Jan. 8 deadline.

The state party has enlisted hundreds of volunteers to call or go door-to-door to contact voters on the list of rejected ballots compiled by the secretary of state’s office.

“Our voter protection operation, including ballot curing efforts, is absolutely central to our mission to help every Georgia voter make their vote count,’ said Maggie Chambers, a party spokeswoman. “Voter protection volunteers help voters every day to take the final step to make sure their vote is counted.”

It’s unclear what, if anything, the Republican Party is doing to cure rejected mail-in ballots. A state Republican Party spokeswoman did not respond to multiple requests for comment. But an official with Republican Secretary of State Brad Raffensperger’s office said she has not seen a lot to suggest the GOP is actively involved in curing ballots.

“In the aftermath of the special elections, we are going to see a lot of scapegoating due to poor planning by the Georgia Republican Party,” said Deputy Secretary of State Jordan Fuchs. “All parties had the ability to create an absentee ballot chase program and an absentee ballot cure program. It’s becoming more clear by the day that only one group came prepared.”

Republican Senators David Perdue and Kelly Loeffler are in tight races against Democratic challengers Jon Ossoff and the Rev. Raphael Warnock in runoff elections Tuesday.

Mail-in ballots and how they are counted have been a hotly-contested issue this fall. The Republican Party has argued in legislative hearings and in lawsuits that Georgia’s system of matching signatures on mail-in ballots is susceptible to fraud. President Trump has made unfounded claims the election in Georgia was stolen from him, largely based on a flawed absentee ballot system that favored Democrats. But those claims have not been matched by evidence. A signature match audit of 15,000 mail-in ballot envelopes in Cobb County conducted by the GBI and Raffensperger’s office released last week found no fraud.

“This audit disproves the only credible allegations the Trump campaign had against the strength of Georgia’s signature match processes,” said Raffensperger.

Still, the noise around the process has intensified focus on absentee ballots and the way they are processed. When the mail-in ballots are received at local election offices, workers compare the signature on the envelope with the signature of a voter’s registration form to ensure they match. If there is no signature or the signature doesn’t match, the offices are required to contact the voters to give them a chance to verify the ballot and have it counted. The deadline is three days after the election.

Cherokee County election officials are among those who have been extra careful in verifying signatures during the runoff, said Anne Dover, the county’s elections supervisor. She said they’ve had to, in part, because the level of scrutiny has intensified.

Overall, the county has mailed more than 600 letters to voters saying their ballots had been rejected. The letter includes an affidavit form with instructions on how to cure the ballot so it will be counted. Just over 300 voters had responded to the forms as of Thursday, with the other 300 or so rejected ballots still outstanding.

Dover said the last thing she wants is to face an audit or state investigation because the office didn’t follow the guidelines. “I’m not going to be accused of not checking signatures,” she said.

In a sign of how contentious absentee ballot verification has become, Dover said criticism has come from Democrats and Republicans. Some Democrats say officials are too aggressive in rejecting ballots, while Republicans have said they aren’t doing enough. One local elected official intentionally signed his ballot differently than his regular signature to try to test the office, Dover said, and then he posted the episode on Facebook.

“We have people trying to trick us,” she said. “We’re looking more closely at the signatures.”

Clara Green, who is among the hundreds of volunteers helping the Democratic Party cure votes, estimates that dating back to the general election she has helped dozens of voters in the metro area.

On Monday, she said she was in Cherokee to help a woman whose signature had been impacted by advanced arthritis. Green said that same day she helped a man who had advanced Parkinson’s and was on home hospice. The man’s family didn’t know the ballot could be cured from their home by emailing an affidavit and photo ID to the local election office.

Another day, Green was in south Fulton assisting an 86-year-old who successfully cast her absentee ballot in the November general election. But her absentee ballot had been flagged in the runoff because of some sort of signature issue.

“The vast majority of times voters didn’t know their ballot had been rejected until I knocked on the door,” she said.

The Democratic Party expects its vote curing activities to intensify through the Jan. 8 ballot deadline.

“It’s important that every vote be counted,” said state Rep. Bee Nguyen (D-Atlanta), who has been reaching out to constituents to help contact neighbors and others in her district whose ballots have been rejected. “In a close election, it’s especially critical.”

Data specialist Jennifer Peebles contributed to this report.