The service has been curtailed amid cost-cutting and efficiency measures ordered by the Trump-appointed new postmaster general as it braces for an unprecedented number of mail-in ballots as a result of the coronavirus pandemic.
The US Postal Service has sent letters to most states warning that millions of ballots cast by mail for November's presidential election may not arrive in time to be counted.
The letters, dated July 29 and delivered to election officials in 46 states and in Washington DC, said that even if voters meet their state deadlines, the Postal Service could not guarantee delivery in time, the Washington Post said on Friday.
They warn that "certain deadlines for requesting and casting mail-in ballots are incongruous with the Postal Service's delivery standards," according to several letters posted on the USPS website following an information request by the newspaper.
"This mismatch 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."
Due to the coronavirus pandemic there are likely to be an unprecedented number of postal votes in what is set to be a very contentious election.
Facing an uphill battle to retain the White House, President Donald Trump has launched a battle against mail-in voting, which he fears would favour Democratic rival Joe Biden.
"Attempts to undermine the election"
Former President Barack Obama has criticised Trump's "attempts to undermine the election," tweeting on Friday that the administration was "more concerned with suppressing the vote than suppressing a virus."
"If you're in a state where you have the option to vote early, do that now," he said on Twitter.
If you're in a state where you have the option to vote early, do that now. The more votes in early, the less likely you're going to see a last minute crunch, both at polling places and in states where mail-in ballots are permitted. Then tell everyone you know.— Barack Obama (@BarackObama) August 14, 2020
Trump openly acknowledged this week that restricting the US Postal Service, including depriving it of resources in the run-up to the election, would impact vote-by-mail efforts.
Clamping down on postal service
He installed Republican fundraiser Louis DeJoy, who previously ran a freight shipping firm, as postmaster general in early May.
DeJoy has clamped down on overtime pay and hiring of mail carriers, which critics say have contributed to slowing deliveries at a time when voting by mail will be critical.
The president has repeatedly claimed that mail-in voting leads to massive fraud, a claim not supported by any evidence from states already offering voting by mail.
Despite the warning letters, the USPS said it was "well prepared and has ample capacity to deliver America's election mail," in a statement quoted by CNN.
"However, the increases in volume and the effect of when volumes were mailed in the primary elections presented a need to ensure the Postal Service's recommendations were reemphasized to elections officials."
Trump – who himself has voted by mail – has made clear he believes that the Democrats will reap more votes if the practice is universal.
Mail-in voting "doesn't work out well for Republicans," he said in April.
Protesters slam US postmaster outside his home
A small group of demonstrators held a noisy protest outside DeJoy's Washington condo amid growing concerns that he is gutting the US Postal Service to help Trump win re-election.
But just as the Postal Service should be gearing up to deal with the deluge, De Joy - a Trump donor who has invested in mail rivals - has pushed through cost-cutting measures that have led to widespread delays.
DeJoy has argued that the changes, including overtime curbs, are needed to return the Post Office to profitability. But demonstrators accused him of sabotage to please Trump, who is trailing in the polls and has claimed - without evidence - that his opponents will use postal voting to cheat him out of office.
Blowing vuvuzelas and banging pots and pans, about 75 people surrounded DeJoy's upscale Washington condo and waved signs saying "Postmaster Saboteur," "LET AMERICA VOTE" and "Deliver DeMail, Depose DeJoy."
The door to DeJoy's condo building was stuffed with mock mail-in ballots. One of DeJoy's neighbours joined in the demonstration, waving from her window at the protesters below.