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".
Critics have blamed the new USPS head - a loyal supporter of President Donald Trump - for a slowdown in deliveries.
A record number of people are expected to vote by mail due to the pandemic.
But on Thursday, Mr Trump said he was blocking additional funding for the USPS to help with election issues, because he opposed mail-in voting.
He has repeatedly said mail-in ballots will lead to voting fraud - and give a boost to his rival Democrat Joe Biden. Experts say the mail-in voting system - which is used by the American military and by Mr Trump himself - is safe from tampering.
Former President Barack Obama strongly criticised what he described as Mr Trump's "attempts to undermine the election", writing on Twitter that the administration was "more concerned with suppressing the vote than suppressing a virus".
Meanwhile, Congress's two top Democrats - Nancy Pelosi and Chuck Schumer - called on the president to stop his "assault" on the postal service and "allow the 2020 election to proceed without his sabotage tactics".
Their comments come as a poll by Axios/ Survey Monkey found that three quarters of Republican voters plan to vote in person, while more than half of Democratic voters plan to use a mail vote.
Private delivery services Fedex and UPS have both rejected calls to help ease the pressure on the postal agency.
Meanwhile, the USPS has reportedly begun removing mail sorting machines - many of which would normally be used to process ballots during the election - according to Vice.