Donald Trump's campaign team has been videotaping voters depositing ballots at drop boxes in Pennsylvania, a key battleground state with 20 Electoral College votes that could be key in deciding the outcome of November's election.
Josh Shapiro, the state's Democrat attorney general, warned the US president's surrogates that surveilling polling stations could be classed as "voter intimidation" and may be "illegal".
“Our entire system of voting is built on your ballot being private and your choice to vote being a personal one,” the state's top prosecutor said on Thursday.
Attorney General Shapiro added: “Pennsylvania law permits poll watchers to carry out very discrete and specific duties — videotaping voters at drop boxes is not one of them".
Linda Kerns, a lawyer for the Trump campaign, confirmed in a letter earlier this week that representatives had been recording voters at drop boxes in Philadelphia, a majority-black Democrat stronghold that Hilary Clinton carried by more than 400,000 votes in 2016.
Ms Kerns claimed the 14 October footage shows three people dropping off as many as three ballots at drop boxes, activity the city's election officials could not confirm was inappropriate under state laws.
She wrote to the state's election chiefs last week requesting they end use the of "unmanned drop boxes", the New York Times repored.
Pennsylvania law, in most cases, requires voters to deliver their own mail-in ballots, Ms Kerns wrote, although it makes an exception for voters with disabilities.
She suggested the images amount to “blatant violations” of state election law and said the campaign would sue unless the city’s election office “commits to remedy this problem immediately.”
She asked for copies of city surveillance video at City Hall, for a list of voters who dropped ballots in the Philadelphia City Hall drop box on 14 October, and that the ballots be set aside “until an investigation can determine whether the ballots were personally delivered” by the voter.
In response, a city lawyer, Benjamin Field, wrote to Ms Kerns on Monday to reject her assumption that the law was violated. Third-party delivery is permitted in certain circumstances, he wrote.
Though the city had forwarded the campaign’s information to the district attorney’s office, Mr Field said, the elections office does not track whose ballots are dropped into particular drop boxes.
In a statement, the office of district attorney Larry Krasner, a Democrat, said it is committed to investigating “any and all” allegations of voter intimidation and harassment.
It comes just days after the supreme court upheld a ruling by Pennsylvania's highest court that extended the state's deadline to accept mail-in ballots, in what has been described as a win for Democrats.It will allow vulnerable voters to avoid large queues at polling stations and give them more time to cast postal ballots.
Due to the ongoing coronavirus pandemic, which has so far claimed some 223,000 US lives, millions of Americans have cast their ballots using dropboxes to avoid potential mail-in ballot delays.
Struggling in the polls, the Trump campaign has throughout the election campaign attempted to restrict access to voting and claimed, without evidence, that mail-in ballots could fall foul to voter fraud.
Earlier this year, the Trump campaign used similar images to those recorded in Philadelphia in a federal lawsuit aimed at banning drop boxes. A judge threw out the case on 10 October.
Philadelphia and many other heavily populated counties in states across the country are using drop boxes to help collect a surge of mail ballots.
On 13 October a federal court reinstated an order by Texas governor Greg Abbott that limits absentee ballot drop-off sites in the state to one per county, a move campaigners said would suppress voting.
Judges on the 5th US Circuit Court of Appeals, each of them appointed by the president, ruled in favour of governor Abbott, saying that his concerns about mail-ballot security trumped those of campaigners, who claim the changes mean hundreds of thousands of Texans would not be able to vote safely due to the coronavirus pandemic.
Texas has traditionally been a Republican stronghold but polling shows the state on a knife-edge this year. According to a Quinnipiac University poll, Mr Trump and Mr Biden are tied on 47 per cent among likely voters.
Additional reporting by Associated Press