The US Postal Service (USPS) has warned that millions of mail-in votes may not arrive in time to be counted on the presidential election day, 3 November.
In letters to states across the country last month, the agency said "certain deadlines... are incongruous with the Postal Service's delivery standards". In a letter to Pennsylvania's secretary of state, the USPS said mail-in ballots requested one week before the 3 November election - allowed under the state's election laws - may not reach their destination on time because the state's deadlines are too tight for its "delivery standards". USPS General Counsel Thomas Marshall said a "mismatch" between Pennsylvania's laws and the mail system's delivery capabilities "creates a risk that ballots requested near the deadline under state law will not be returned by mail in time to be counted under your laws as we understand them".
Critics have blamed the new USPS head - a loyal supporter of Trump - for a slowdown in deliveries. Trump said he was blocking additional funding for the USPS to help with election issues, because he opposed mail-in voting.
A record number of people are expected to vote by mail due to the pandemic.
Secretary of State Kathy Boockvar asked the state's supreme court to allow ballots to be counted as long as they were received up to three days after the election. Currently, votes are discarded if they are received after election day.
Pennsylvania is a battleground state, which Mr Trump won by less than 1% in the 2016 election. Other battleground states, including Florida and Michigan, also received letters, according to US media reports.
The Democratic governor in Pennsylvania's neighbouring New Jersey announced on Friday that the state would pre-emptively send ballots to every registered voter in the state. The process of sending out ballots is known as universal mail-in voting, and has been adopted in nine other US states.