Democrats have accused Donald Trump's postmaster general of trying to sabotage the postal service ahead of what is expected to be an explosion of mail-in voting for the Nov 3 election.
Mr Trump has railed against the expansion of mail-in voting, claiming the election is a "catastrophe waiting to happen," that it will be fatally undermined by fraud, and deliveries will not be able to cope with the number of ballots.
But Democrats have accused him of setting up the postal service as a scapegoat to blame if he loses.
At the centre of the storm is Louis DeJoy, who became postmaster general in June.
Mr DeJoy, 63, a North Carolina businessman, donated more than $2 million to Mr Trump and Republicans over the last four years.
His wife has been nominated by Mr Trump to be the next US ambassador to Canada.
In the last few months he introduced a swathe of cost-cutting measures.
That included eliminating overtime for hundreds of thousands of postal workers, restricting the number of delivery trips, and reducing equipment at mail processing plants.
On Friday he announced more sweeping changes, displacing or reassigning two dozen top postal executives.
Democrat congressman Gerald Connolly, chair of the House subcommittee responsible for postal oversight, said: "This is deliberate sabotage to disrupt mail service on the eve of the election - an election that hinges on mail-in ballots.
"Postmaster General DeJoy should be focused on ensuring delivery standards, not this Trojan Horse reorganisation.”
Nancy Pelosi and Chuck Schumer, the Democrat leaders in Congress, met with Mr DeJoy this week in what was described as a "heated" meeting.
The Democrat leadership said: "We believe these changes, made during the middle of a once-in-a-century pandemic, now threaten the timely delivery of mail - including medicines for seniors, pay cheques for workers, and absentee ballots for voters."
Mr Schumer said he told Mr DeJoy that "elections are sacred" and urged him not to impose cutbacks "at a time when all ballots count."
He added: "We must resolve this in a way that allows mail to be delivered on time for the election and for the necessities that people need."
Mr DeJoy, who was appointed by the Postal Service’s board of governors not Mr Trump, has vociferously denied any intent to disrupt voting.
He said: “While I certainly have a good relationship with the President of the United States, the notion that I would ever make decisions concerning the Postal Service at the direction of the President, or anyone else in the administration, is wholly off-base.
"I serve at the pleasure of the governors of the Postal Service, a group that is bipartisan by statute and that will evaluate my performance in a nonpartisan fashion."
The US postal service lost $2.2 billion in the second quarter of the year.
Mr DeJoy said that it had a "broken business model" and "without dramatic change there is no end in sight and we face an impending liquidity crisis."
Mr DeJoy, the first postmaster general in nearly two decades who was not a career postal employee, has reportedly been nicknamed "Louis DeLay" by some workers as reports of delayed post mount.
Records for mail-in voting are expected to be shattered in the election.
In the state of Iowa a Monmouth University poll found 40 per cent of voters were "very likely" to vote by mail.