A Pennsylvania federal appeals court has ruled that mail-in ballots received without accurate handwritten dates on the outside of envelopes are not valid, a ruling that will have a significant impact on this year's elections in the key battleground state.

The U.S. Court of Appeals for the Third Circuit ruled 2-1 on Wednesday, overturning a lower court’s November decision.

The lower court had ruled that even without the proper dates, mail-in ballots should be counted if received on time. The court said that "trivial paperwork" errors disenfranchised voters and violated the Materiality Provision of the U.S. Civil Rights Act of 1964, which states that immaterial errors or omissions should not be used to prevent voting.

Former President Donald Trump has railed against mail-in ballots for years. (Michael M. Santiago/Getty Images, left, Ed Jones/AFP via Getty Images, right.)

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But in Wednesday’s federal appeals court’s opinion, Judge Thomas Ambro wrote that under a state legislature passed law, mail-in voters must write the date on their envelopes in order to make their vote effective.

"The Supreme Court of Pennsylvania unanimously held this ballot-casting rule is mandatory; thus, failure to comply renders a ballot invalid under Pennsylvania law," Ambro wrote.

The Materiality Provision "only applies when the State is determining who may vote," Ambro wrote.

Under a state law passed in 2019, Pennsylvania voters must "fill out, date and sign the declaration printed on [the] envelope" before returning their ballot.

Proponents of mail-in ballots argue it makes voting more convenient, as well as being easier for senior citizens and people with disabilities to vote. However, some Republicans say that the process raises serious election integrity issues, while former President Donald Trump has blasted the system as "totally corrupt" and blames it, in part, for his 2020 election loss.

In Pennsylvania, Democrats have been far more likely to vote by mail than Republicans.

Chester County election workers process mail-in and absentee ballots during the 2020 presidential election. (AP Photo/Matt Slocum)

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"This is a crucial victory for election integrity and voter confidence in the Keystone State and nationwide," Michael Whatley, the chair of the Republican National Committee said in a statement.

"Pennsylvanians deserve to feel confident in the security of their mail ballots, and this Third Circuit ruling roundly rejects unlawful left-wing attempts to count undated or incorrectly dated mail ballots."

The American Civil Liberties Union of Pennsylvania, which helped represent groups and voters who challenged the date mandate, said that voters will lose as a result of the ruling.

Michael Whatley, the chair of the Republican National Committee, welcomed the ruling. (Robert Willett/The News & Observer/Tribune News Service via Getty Images)

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"If this ruling stands, thousands of Pennsylvania voters could lose their vote over a meaningless paperwork error," said Mike Lee, the executive director of the ACLU in Pennsylvania.

"The ballots in question in this case come from voters who are eligible and who met the submission deadline. In passing the Civil Rights Act, Congress put a guardrail in place to be sure that states don’t erect unnecessary barriers that disenfranchise voters. It’s unfortunate that the court failed to recognize that principle."

In the 2022 midterms, more than 7,600 mailed ballots in 12 counties were tossed because their outer envelopes lacked dates or had incorrect dates, according to the decision.

The Associated Press contributed to this report.