"Biden and Harris fully exhibit the experience, dedication, thoughtfulness and steady hands that will work to ensure that letter carriers and working families are put first ... The Postal Service must not be allowed to fail," said a letter from the National Association of Letter Carriers, which represents the 300,000 active and retired USPS workers.
In 2016, the union endorsed former Secretary of State Hillary Clinton.
Former President Barack Obama took to Twitter Friday to criticized the Trump administration over the USPS changes.
"Everyone depends on the USPS ... They can't be collateral damage for an administration more concerned with suppressing the vote than suppressing a virus," he said.
Everyone depends on the USPS. Seniors for their Social Security, veterans for their prescriptions, small businesses trying to keep their doors open. They can't be collateral damage for an administration more concerned with suppressing the vote than suppressing a virus.— Barack Obama (@BarackObama) August 14, 2020
In an interview on Fox Business Network, Trump explicitly noted two funding provisions that Democrats are seeking in a relief package that has stalled on Capitol Hill. Without the additional money, he said, the Postal Service won't have the resources to handle a flood of ballots from voters who are seeking to avoid polling places during the coronavirus pandemic.
"If we don't make a deal, that means they don't get the money," Trump told host Maria Bartiromo on Thursday. "That means they can't have universal mail-in voting; they just can't have it."
Trump's statements, including the false claim that Democrats are seeking universal mail-in voting, come as he is searching for a strategy to gain an advantage in his November matchup against Joe Biden. He's pairing the tough Postal Service stance in congressional negotiations with an increasingly robust mail-in-voting legal fight in states that could decide the election.
The USPS approved price increases on its commercial domestic parcels "in response to increased expenses and heightened demand for online shopping package volume due to the coronavirus pandemic." Retail customers will not be affected.
Postmaster General Louis DeJoy has said that the agency is in a financially untenable position, but he maintains it can handle this year's election mail. A major donor to Trump and other Republicans, DeJoy is the first postmaster general in nearly two decades who is not a career postal employee.
"Although there will likely be an unprecedented increase in election mail volume due to the pandemic, the Postal Service has ample capacity to deliver all election mail securely and on-time in accordance with our delivery standards, and we will do so," he told the Postal Service's governing board last week.
Memos obtained by The Associated Press show that Postal Service leadership has pushed to eliminate overtime and halt late delivery trips that are sometimes needed to ensure mail arrives on time, measures that postal workers and union officials say are delaying service. Additional records detail cuts to hours at post offices, including reductions on Saturdays and during lunch hours.
The Associated Press contributed to this report.