HARRISBURG, Pa. (AP) — Pennsylvania's Supreme Court ruled unanimously Friday on a key concern surrounding an avalanche of mailed ballots, prohibiting counties from rejecting them if the voter's signature on it does not resemble the signature on the voter's registration form.

Two Republican justices joined five Democratic justices in the decision.

The verdict was a victory for the state's top election official, Kathy Boockvar, a Democrat who had asked the court to back her up in a legal dispute with President Donald Trump's campaign and Republican lawmakers.

“County boards of elections are prohibited from rejecting absentee or mail-in ballots based on signature comparison conducted by county election officials or employees, or as the result of third-party challenges based on signature analysis and comparisons,” the justices wrote.

Trump and former Vice President Joe Biden are locked in a battle to win Pennsylvania's 20 electoral votes. With Democrats voting by mail at an almost 3-to-1 rate over Republicans, the prospect of disqualified ballots poses a greater threat to Biden's candidacy.

In her court filing, Boockvar had said that any such rejections pose “a grave risk of disenfranchisement on an arbitrary and wholly subjective basis,” and without any opportunity for a voter to verify their signature before their ballot is disqualified.

Republican lawmakers and the Trump campaign had argued that the law is clear that election officials must compare the information on the mail-in ballot envelope, including a voter’s signature, to a voter’s information on file to determine a person’s qualifications to vote.

But the justices disagreed, as did a federal judge in a separate case brought earlier by Trump's campaign. Both said that the law on mail-in ballots makes it clear only that the ballot envelope requires the voter's signature, but not a matching signature.

Pennsylvania has no law mandating that voters get an opportunity to fix an irregularity with their ballot before it is disqualified, and discussions about it in the Legislature in recent weeks deadlocked in a wider partisan fight shadowed by the presidential election.

In a statement, Pennsylvania's attorney general, Democrat Josh Shapiro, called the decision a “win for voters.”

Voters who use a mail-in ballot have their identity verified in their initial application, often using a driver's license number, he said.

“Pennsylvania’s voter identification system is safe and secure,” he said. “We are protecting every eligible vote and ensuring each is counted.”

Senate Majority Leader Jake Corman, R-Centre, accused Boockvar of undermining election security provisions in state law and said lawmakers never thought it could be interpreted to render “signatures required on the mail-in ballots being meaningless.”

“People voting in person are now being held to a higher standard than those who mail in their ballots,” he said in a statement.

He also said it is “astonishing” that courts endorsed the interpretation that matching signatures are required to vote in person, “but mail-in voting is officially a free-for-all.”

The decision comes amid a surge in mail-in voting and rising concerns that tens of thousands of mailed ballots will be discarded in the presidential election over a variety of technicalities.

Some voters say that signing on a digital screen when getting or renewing their driver's license is awkward and results in a signature that doesn't resemble theirs on paper.

County election officials say people’s signatures change over time, with age or medical conditions.

They also say questions about whether a voter’s signature is valid have historically been rare and, because of that, there has never before been a debate about it until now, with coronavirus concerns fueling interest in voting by mail under a year-old law that vastly expanded it.

In the Nov. 3 presidential election, more than 2.9 million voters in Pennsylvania have requested mail-in ballots, more than 10 times the number in 2016′s presidential election and many of them from people who have never voted by mail before.

About half have been returned to election offices, according to state election officials.

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Follow Marc Levy on Twitter at www.twitter.com/timelywriter.

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AP’s Advance Voting guide brings you the facts about voting early, by mail or absentee from each state: https://interactives.ap.org/advance-voting-2020/