Amid one of the most hotly contested presidential elections in generations — and one in which the Republican presidential candidate has claimed that the balloting process if "rigged" — thousands of New Jersey voters are at risk of not having their votes counted in November.

Clerks in at least three Republican-heavy counties have reported that hundreds of mail-in ballots were erroneously returned to voters instead of being delivered to the county election boards.

The problem is with new Post Office equipment that was unable to properly read the mailing address on some — but not all — ballot envelopes mailed by voters.

Voters and county election officials began noticing the problem in late September, prompting counties to send out subsequent batches of newly designed mail-in ballots with an updated envelope barcode that postal officials say would prevent the error.

According to Somerset County Clerk Brett Radi, postal officials told him that post office workers were instructed to keep an eye out for mail-in ballots and to place them in a new envelope with the proper election board address rather than returning them to voters.

But on Tuesday, Radi said he spoke to a voter who received a returned ballot that she had mailed on Friday.

"I am extremely concerned that this happened," said Radi.

Radi is especially worried about voters who "put those ballots in good faith in the mail before going out of the state" for vacation or other reason. Those voters may not discover until after the election that their ballots were returned — and then it will be too late.

A spokesman for the U.S. Postal Service in North Jersey said the problem was with the design of the ballots.

"The Somerset County Clerk’s office addressed the issue and is now printing envelopes that are compatible with our automated processing equipment," spokesman George B. Flood said Tuesday in an email. "We continue to monitor this situation, identify those pieces that were printed prior to the fix, and ensure all ballots are delivered to our county clerks’ offices in a timely manner."

In New Jersey, mail-in ballots have to be received by the county election boards no later than Election Day in order for them to be counted.

Radi said he asked postal officials to scour post office boxes and vacation mail holds to make sure any improperly returned ballots aren't just sitting there.

Clerks in Hunterdon and Monmouth also reported similar problems with their ballots.

Officials in Middlesex, Morris and Bergen counties, meanwhile, said Tuesday that voters had not complained about returned ballots.

Voters who want to make sure that their ballot was received should call their county's Board of Elections or the state hotline 1-877-NJVOTER .

Oct. 18 is the deadline to register to vote in the Nov. 8 election.

County officials told The Associated Press Tuesday they have seen no evidence of voter fraud and are not concerned the system is compromised, despite unsubstantiated claims by Republican presidential candidate Donald Trump that the system is rigged.

"I am a Republican and I have 1,000 percent faith in the New Jersey election system," Hunterdon County Clerk Mary Melfi said.

While Republican Gov. Chris Christie, a top Trump surrogate who chairs his transition team, has scaled back his public comments on the election, he has said that he doesn't see any evidence of vote rigging.

"I don't think there's danger," Christie said in August of potential vote rigging. "I think the American people ... have faith in the efficacy of our election system."

Lt. Gov. Kim Guadagno, the state's secretary of state who is in charge of elections, told the AP the state Office of Homeland Security is monitoring the situation. She declined to speak further.

A state Homeland Security official on Wednesday said the agency works with the Division of Elections "to proactively assess and enhance other controls that protect New Jersey’s election processes."

New Jersey is not considered a battleground state in the presidential contest and reliably votes Democratic in national races, although turnout in a presidential race may affect down-ballot contests for congressional, county and municipal races.

There are 5.7 million registered voters, according state figures released in October, including 2.4 million unaffiliated voters, 2 million Democrats and 1.2 million Republicans.

The Associated Press contributed to this report.

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Sergio Bichao is deputy digital editor at New Jersey 101.5. Send him news tips: Call 609-438-1015 or email sergio.bichao@townsquaremedia.com.