PHILADELPHIA — In what appears to be an organized effort, right-leaning activists have challenged the mail ballot applications of hundreds of voters in suburban Philadelphia in recent days, claiming their targets no longer live at the addresses where they are registered to vote.

But voting rights advocates broadly dismiss the effort as baseless, legally invalid, and born of a misunderstanding of government data.

While they predict most of the challenges will be swiftly rejected, they say the campaign is yet another instance of a loose network of right-wing organizers billing themselves as “election integrity” advocates sowing confusion about state voting laws and creating headaches for elections administrators already bombarded by misinformation surrounding the voting process.

“Add it to the pile,” Bucks County spokesperson James O’Malley lamented when asked about the challenges to 191 mail ballot applications administrators received in that county. He said the county’s board of elections would review them in the proper course.

The Bucks County challenges were all filed Friday by a single person, O’Malley said, though he declined to identify that individual.

Officials in Chester and Delaware Counties reported similar attempts to file challenges there — all following a similar pattern and in some cases using the same boilerplate language.

“We are very concerned about the validity and intent of these challenges, deliberate abuse of the election system, and the time and energy it will take for our local election officials to address them,” said Susan Gobreski, president of the League of Women Voters of Pennsylvania.

According to the American Civil Liberties Union, which has been tracking the effort, filers began presenting “hundreds of cookie-cutter challenges” late last week that appear to be “virtually identical in format and content.”

Each one asserts that the challenged voter is not eligible because they purportedly match with a name in the U.S. Postal Service National Change of Address database, suggesting that at some point someone with that name filed to receive mail at a new address.

The activists say that means it’s likely the voter no longer lives at the address at which they’d registered to vote.

But Witold Walczak, legal director of the ACLU of Pennsylvania, said that database is notoriously prone to error and can’t reliably be used to determine whether a voter is ineligible.

For instance, he said, a registered voter might temporarily seek to change their mailing address for any number of reasons — like a temporary job relocation, military deployment, or college — while still remaining eligible to vote in their home county.

“The standard for being on that NCOA list is different from the standard for being an eligible voter,” he said.

What’s more, federal law prohibits these kinds of mass-challenges to voter eligibility within 90-days of an election.

Nonetheless, one activist attempted to drop off a stack of challenges in Delaware County last week. She was turned away for failing to properly fill out the required paperwork, officials there said. Activists have until a Friday deadline to try again.

Another challenged the eligibility of some 212 voters in Chester County, according to county spokesperson Becky Brain. The challenges targeted voters of all party registrations but have skewed heavily toward registered Democrats because they are more likely to vote by mail.

Who is behind the effort is unclear.

Walczak said the ACLU first learned of the campaign earlier this year when it began receiving complaints from voters who’d received anonymous letters questioning their eligibility.

The missives — ominously signed “a friend from Pennsylvania” — cited the change of address database and encouraged the recipients to deregister themselves in Pennsylvania.

Officials identified the person who dropped off the Chester County challenges as Diane Houser.

Houser, 73, of Downington, did not respond to requests for comment Tuesday. But she was the lead plaintiff in a lawsuit filed this summer by a group called United Sovereign Americans that challenged the accuracy of the state’s voter rolls. That case, which state election officials have panned as frivolous and laden with conspiracy claims, remains pending in federal court in Harrisburg.

Meanwhile, right-wing activist groups like True the Vote and the Election Integrity Network — an organization led by Cleta Mitchell, an adviser to the Trump campaign — have previously relied on the National Change of Address database in efforts to purge voter rolls in Pennsylvania and in other states.

Both parties have even developed software meant to assist grassroots conservatives to use that data at scale to identify potential targets for eligibility challenges.

However, it was not clear whether any of the activists who’d filed the recent challenges in Chester, Bucks, and Delaware Counties were associated with or inspired by those groups.

So far the challenges have been limited to only those three counties. However, voting rights organizations are concerned more could be coming before the Friday deadline for challenges to mail ballot applications.

On Monday, the ACLU preemptively sent letters to the boards of elections in all of Pennsylvania’s 67 counties advising them of the issues with the change of address database and the law surrounding mass-challenges to voter eligibility.

“The proper course of action for any county receiving these challenges,” it read, “is to reject them as insufficient.”