The effort - which appears to be spearheaded by just a handful of individuals - is seeking to toss out ballots from overseas voters and to challenge the eligibility of people who, at some point, had requested to have their mail forwarded to another address.
More than a dozen counties received these challenges, according to CNN's outreach to local and state officials.
Some election boards already have set aside the complaints as meritless, but they come as officials are scrambling to carry Tuesday's consequential election in a state that could determine the outcome of the race for the White House.
"It's going to make voters nervous and it's creating totally - frankly - BS work for election boards when they're trying to do an important job," said Ari Savitzky, a senior attorney for the ACLU.
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In some counties, the challenges were filed minutes before the Friday deadline for such objections. Bucks County, for instance, received nearly 1,200 challenges to overseas voters about 15 minutes before the 5 p.m. deadline Friday deadline, according to county spokesperson Jim O'Malley.
Challenges filed in Bucks County - which includes Philadelphia's northeastern suburbs - were all filed by Pennsylvania state Sen. Jarrett Coleman, O'Malley said. Coleman also filed 519 challenges to overseas voters in Lehigh County, according to Tim Benyo, that county's elections chief.
Coleman, a Republican who represents parts of both counties, did not return CNN's request for comment. A fellow state Republican senator, Sen. Cris Dush, filed challenges to 77 voters in Centre County; his office declined to comment.
A statement from the office of Pennsylvania's top election official, Secretary of the Commonwealth Al Schmidt, described the "bad-faith mass challenges" as a "coordinated effort" that were "based on theories that courts have repeatedly rejected."
At least 3,000 Americans abroad who applied to vote by mail in Pennsylvania had their ballots challenged. Hundreds of mail voters were also targeted with challenges that appear to have been assembled using US Postal Services' database for people who have requested a change of address.
Republicans and outside conservative groups have launched legal assaults against overseas ballots in multiple battleground states, including Pennsylvania, after Democrats touted efforts to turn out citizens who live abroad.
Though the overseas vote has long been associated with military voters, civilian expats overseas now outnumber those voters. In the 2020 presidential election, there were more than 28,000 overseas voters in 2020 in Pennsylvania - a state that President Joe Biden won by fewer than 90,000 votes.
Counties moving quickly to resolve challenges
Chester County disposed of the 212 challenges filed last week based on US Postal Service data at a Friday hearing where several individuals who had been challenged confirmed their eligibility to vote in Pennsylvania, with some citing their current status as students at out-of-state colleges or graduate schools.
In Lycoming County, the mail ballots of 71 overseas voters were challenged by Karen DiSalvo, who previously represented a group of Republican congressmen from Pennsylvania that filed an unsuccessful lawsuit seeking more stringent verification procedures for overseas ballots. Pennsylvania officials said that the lawsuit risked disenfranchising "tens of thousands" of overseas voters, including those in uniform.
In an email to CNN, DiSalvo said she had filed the challenges against non-military members overseas who had applied for mail ballots but were not registered to vote in the county.
In Washington and Beaver counties, a total of 265 mail ballot applications were challenged by Charles Faltenovich, who sits on the steering committee of PA Fair Elections, which says it aims for "secure elections" in the state and was involved in the overseas ballot lawsuit in Pennsylvania.
Faltenovich told CNN his challenges were not connected to the group and that he targeted applicants who indicated on their ballot application that they are currently and indefinitely out of the country and that they are not a member of the military.
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While states can enforce voter registration requirements for overseas citizens, Pennsylvania law doesn't appear to explicitly address this category of voters, said Michael Morley, an election law expert at Florida State University. And federal law makes clear that these voters have the right to continue voting in the state they last resided in when they lived in the United States, he added.
"In the absence of a registration requirement that applies to (overseas voters), election officials are required to fill their timely submitted absentee ballot requests," Morley said.
The deadline for county election boards to resolve these challenges is November 8, which is when the Lycoming County Board of Elections has scheduled its hearing on the challenges filed there. If a voter is disqualified, her or she has the option to appeal in Pennsylvania court, as does a challenger whose challenge is rejected.
DiSalvo told CNN she will "likely appeal the challenges in court" if they are rejected by the county board.
CNN's Danya Gainor contributed to this report.
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