The US Postal Service has suspended new policies that were decried as an attempt to sabotage the 2020 election.
Postmaster General Louis DeJoy said he would reverse operational changes that critics say would hamper postal voting.
The U-turn comes as Mr DeJoy is due to testify to Congress and at least 20 states were preparing to sue.
There is a fierce debate over postal funding in 2020, as record numbers of Americans are expected to vote by mail due to the pandemic.
The US Postal Service (USPS) under Mr DeJoy had begun what it said were cost-cutting measures in recent months.
Policies that were begun under Mr DeJoy included removing mail boxes, cancelling delivery runs and closing down sorting centres.
In a sharp reversal, Mr DeJoy has now said that post office hours would not be cut, and post boxes and sorting machines would stop being removed.
Mr DeJoy, a former Republican donor, also said overtime pay would continue to be approved to ensure deliveries arrive on time.
"To avoid even the appearance of any impact on election mail, I am suspending these initiatives until after the election is concluded," Mr DeJoy said in a statement.
Why the U-turn?
A week ago, Donald Trump said he had no interest in any additional funding for the US Postal Service, lest the money be used to help process mail-in voting. It was all part of his ongoing, and largely unfounded, campaign against the expanded use of postal ballots to minimise the risk of spreading coronavirus.
By this Monday, the president tweeted that he wanted to "save the post office" and told a crowd in Minnesota that he would "strengthen" the service.
And now, his postmaster general has said the agency will stop taking out postal boxes and limiting delivery routes.
It turns out the Postal Service is pretty popular. A Morning Consult poll found 80% of Americans have a positive view of it. The elderly use it to receive prescription drugs. For rural residents, it's a lifeline to the rest of the world.
Whether the recent moves were a misconstrued part of a long-planned change or, as some on the left suspect, the result of a larger conspiracy, the White House concluded that there was only one way out - retreat.
What's the reaction?
The development comes as the row over the politicisation of the most popular US government agency has become a top issue in the 2020 presidential campaign.
Over the weekend, former President Barack Obama - in what was regarded as his most high-profile criticism of his successor to date - accused Mr Trump of trying to "actively kneecap" the postal service.
Defenders of the changes said they were necessary to help the USPS get out of financial debt. Its budget shortfall has risen to $160bn (£122bn) amid a decade-long decline in mail volume.
However, Mark Dimondstein, the president of the American Postal Workers Union which represents more than 200,000 postal employees, told Fox News on Tuesday that the changes "are truly slowing down mail, the customers see it... the postal workers see it - mail is getting all backed up".
Nancy Pelosi, the Speaker of the House, cheered the postmaster's volte-face on Tuesday, telling reporters: "They felt the heat and that's what we were trying to do, make it too hot to handle." On Sunday, Ms Pelosi had recalled the House from a recess in order to investigate the USPS policies.
Mr DeJoy, a major political donor who was appointed by Mr Trump to lead the USPS in May, is due to testify to a Republican-led Senate committee on Friday, and then to a Democrat-led House committee on Monday.
What's Trump said?
Last week, President Trump said he rejected a funding boost for the USPS to shore up a predicted influx mail-in voting, claiming without evidence that it would lead to voter fraud and help Democrats.
Mr Trump has also suggested delaying the election, which he does not have the power to do, to stop postal ballots leading to "inaccurate and fraudulent" results.
Voting by mail is not new to the US. According to Reuters, approximately one in every four voters cast ballots by mail in 2016.
Critics say people could vote more than once via absentee ballots and then again in person, though numerous nationwide and state-level studies over the years have found no evidence of widespread fraud.
But these are rare incidents, and the rate of voting fraud overall in the US is between 0.00004% and 0.0009%, according to a 2017 study by the Brennan Center for Justice.