A federal appeals panel has set up a potential U.S. Supreme Court battle about Pennsylvania's mail-in ballots that could play a role in determining who wins this year's presidential election and other races in the key swing state.

Mailed ballots that arrive on time but in envelopes without dates handwritten by Pennsylvania voters or with incorrect dates should not be counted, a three-judge panel of the 3rd U.S. Circuit Court of Appeals ruled Wednesday. Their 2-1 decision strikes down a lower court ruling.

The main legal issue surrounding what are often called "undated ballots" is whether not tallying them violates the Civil Rights Act of 1964, which says a person's right to vote cannot be denied for "an error or omission" that is "not material" in determining voting eligibility.

A current, handwritten date on the return envelope is required by Pennsylvania state law, but that date is not used to confirm if a person is eligible to vote. For past elections, the final vote tallies by county election officials have included ballots arriving in undated or misdated return envelopes.

In the panel's majority opinion, 3rd U.S. Circuit Judge Thomas Ambro said that what's known in legal circles as the materiality provision "only applies when the State is determining who may vote."

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