HARRISBURG, Pa. (AP) — A technical requirement that Pennsylvania voters write accurate dates on the exterior envelope of mail-in ballots was again the subject of a court proceeding on Thursday as advocates argued the mandate unfairly leads to otherwise valid votes being thrown out.

A five-judge Commonwealth Court panel heard about two hours of argument in a case that was filed in May, even though the date requirement has been upheld both by the state Supreme Court and the 3rd U.S. Circuit Court of Appeals.

The case was brought by the Black Political Empowerment Project, Common Cause and allied advocacy groups against the secretary of state and the elections boards in Philadelphia and Allegheny County, which includes Pittsburgh. They argued that enforcing the date requirement infringes upon voting rights and that none of the prior cases on the topic directly ruled whether it runs afoul of the state constitution’s Free and Equal Elections Clause.

The number of potentially invalid ballots at stake is a small fraction of the electorate, in the range of 10,000 or more across Pennsylvania in prior elections, and those voters tend to be comparatively older. Democrats have embraced voting by mail much more than Republicans since it was widely expanded in Pennsylvania in 2019 — months before the COVID-19 pandemic — as part of a legislative deal in which Democrats got universal mail-in voting while GOP lawmakers obtained an end to straight-ticket voting by party.

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More than a third of ballots cast in this year’s state primary election were by mail, according to the lawsuit.

Judge Patricia McCullough, a Republican on the panel, asked what authority Commonwealth Court has over the legislatively enacted rule.

“Can this court just come in and change the law because it wasn’t the best thing they should have written or we don’t think it has a purpose? Is that a grounds for us to change or declare something to be invalid?” she asked.

John M. Gore, a lawyer for the state and national Republican Party groups that are fighting the lawsuit, said the court would only have grounds to do so if the procedure was “so difficult as to deny the franchise.” He argued to the judges that the dating requirement is not so onerous that it denies people the right to vote.

The dates serve as a backstop, Gore said, providing evidence about when ballots were completed and submitted. The mandate also “drives home the solemnity of the voter’s choice” to vote by mail, and could help deter and detect fraud, he said.

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County elections officials say they do not use the handwritten envelope dates to determine whether mail-in votes have been submitted in time. Mail-in ballots are generally postmarked, elections officials process and time-stamp them, and the presence of the ballots themselves is enough evidence to show that they arrived on time to be counted before the 8 p.m. Election Day deadline.

Among the issues before the court panel is whether throwing out a portion of the 2019 voting law would trigger a provision under which the entire law must also be thrown out.

Mail-in ballots, and the dating requirement in particular, have spawned several legal cases in Pennsylvania in recent years. Earlier this year, the 3rd U.S. Circuit Court of Appeals upheld the mandate for accurate, handwritten dates, overturning a district judge’s decision.

The Pennsylvania Supreme Court ruled two years ago that mail-in votes may not count if they are “contained in undated or incorrectly dated outer envelopes.” The justices had split 3-3 on whether making the envelope dates mandatory under state law would violate provisions of the U.S. Civil Rights Act of 1964, which states that immaterial errors or omissions should not be used to prevent voting.

During the April primary, redesigned exterior envelopes reduced the rate of rejected ballots, according to state elections officials.