The Republican National Committee sent out a mailer in a key state in the battle for the Senate in the upcoming midterm elections that erroneously informed voters about how to vote absentee.

The mailer told voters in Montana, where incumbent Sen. Jon Tester (D) is engaged in a competitive fight against Republican challenger Matt Rosendale, that absentee ballots could be postmarked until the day before the election as long as they were received within 10 days of Nov. 6.

But election officials told local reporters that the state’s rules stipulate that ballots must be received by 8 p.m. on Election Day, regardless of the postmark.

The RNC said the error was a mistake it was working to fix.

“We regret the unintentional error we made in a recent mailing,” spokesman Rick Gorka said in a statement. “We are contacting all voters who received the mailing through live phone calls and we are sending new mailers to each voter providing clarification.”

Election officials have been sounding alarms about the mailers after a few of them received them, the Billings Gazette reported.

The state’s political practices commissioner, Jeff Mangan, told Montana Public Radio that Republican Party officials have apologized to state officials for the mailer.

“They said it was an oversight, that it was not meant to mislead any Montana voter, and it was their intent to distribute a new mailer with the correct information,” Mangan said.

Mailers with incorrect information because of errors or typos are a somewhat regular occurrence in the run-up to elections, but they can also be a tool of bad-faith actors working to affect voting outcomes in tightly contested elections.

Earlier this month, a mailer sent by a group called the New American Jobs Fund incorrectly told 90,000 residents that they were not registered to vote, according to KTVQ. In Missouri, the state Republican Party sent mailers to 10,000 voters with incorrect information about when their absentee ballots were due. Party officials said that the mailers were part of a get-out-the-vote effort sent to likely Republican voters.

Voting rights has become a tense national issue as Republicans in states across the country have sought to implement laws with more requirements to vote. In Georgia, Republican Brian Kemp, the secretary of state who is also running to be governor, has come under criticism in recent weeks after it was revealed by the Associated Press that his office stalled more than 53,000 voter applications, the majority of them from African Americans, under the state’s “exact match” law.

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