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"..
A record number of people are expected to vote by mail due to the pandemic, BBC reported.
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.