About 30% of voters, mostly Democrats, cast postal ballots in the general election two years ago.
However, several states like Oregon, Washington, and Colorado rely heavily or entirely on mail ballots for their elections, requiring Republicans to use the system as well.
Seven Republican state election officials warned the justices, external that allowing significant changes to mail ballot rules so close to the midterm elections would “lead to mistakes, delays, and confusion”.
Local election officials told the BBC this week they were proceeding as normal but had begun drafting contingency plans in case the court upended existing mail ballot procedures.
The rule changes would have resulted in “thousands of people disenfranchised” in Cherokee County, Georgia, its election director Anne Dover told the BBC. The heavily Republican county voted 69% for Trump in the 2024 election, in which about 10,000 of its residents voted by mail, according to Dover.
Monday’s decision earned praise from legal groups who have challenged other aspects of the Trump agenda.
“Today’s Supreme Court decision prevents the US Postal Service from sowing chaos in our elections,” Trevor Potter, president of Campaign Legal Center, said in a statement.
Since returning to the White House, Trump has sought to restrict voting by mail. While he himself has voted by post, he has long claimed this method allows non-citizens to cast ballots and is prone to fraud.
In March’s executive order, the president directed USPS to introduce unique barcodes on postal ballot envelopes.
State and local election officials would also be required to supply information to an online portal about voters who receive mail ballots.
Twenty-three states and Washington DC sued to stop Trump’s order.
The postal workers’ union also said clerks could not be trained in time for the election.
In court filings, the Trump administration maintained the new USPS rules were constitutional.
It said states “cannot choose to use the federal mails to carry out their elections but then insist that their election-related mail is somehow exempt from the Postal Service’s rule-making authority, conferred by Congress”.
Boston-based Judge Indira Talwani imposed an injunction on 4 September blocking Trump’s order. She ruled that implementing the measures close to November’s elections could disenfranchise voters.
Last week, a US appeals court refused to put Judge Talwani’s order on hold.
On Sunday, another federal judge, Judge Carl Nichols, ruled against Trump’s order.
Some states have already begun distributing mail-in ballots ahead of the midterm elections, which will decide which party controls Congress.
Some states that had not yet sent ballots had already printed their election materials, and changing their voting materials would have required starting the process anew.
“Hypothetically, if we did have to implement, it would have a huge impact, and it would be very costly. At this point, we already have our envelopes printed,” Josh Zygielbaum, the Adams County, Colorado clerk and recorder, told the BBC the day before the Supreme Court ruled.
Colorado conducts its elections by mail, and Zygielbaum had been creating contingency plans in the event its voting system had been disrupted.
The Supreme Court decision is not the first setback on postal ballots for Trump.
In June, the justices ruled that states may count ballots postmarked by election day, rejecting the Trump administration’s push to block votes being tallied when they are received after polls close.
Trump has previously made false claims that he lost the 2020 election owing to widespread electoral fraud.
