The decision allows states to continue sending out mail ballots under the same processes they've used for years.
Republicans are locked in a tight battle to retain control of Congress in the mid-terms. Restricting mail-in ballots would stand to benefit Republicans given that Democratic voters disproportionately use mail-in ballots, according to various surveys.
Justices Samuel Alito and Clarence Thomas publicly dissented from the brief order on Monday.
The Trump administration had asked the court, once again at the centre of a roiling political controversy, to clear the way for restrictions before the pivotal November contests for control of Congress. The case has major implications because nearly one-third of the country votes by mail.
The Supreme Court majority did not detail its reasoning, as is typical on the high court's emergency docket. Alito, meanwhile, said in the dissent that the Postal Service "has broad authority to regulate the mail" and likely does have the power to enforce Trump's restrictions.
Justice Brett Kavanaugh agreed that the restrictions should not go into effect for the mid-terms but indicated he might rule in favour of the Trump administration if the issue comes back before the court.
Election officials have said there was simply no way to carry out a complete overhaul in the weeks before the mid-terms. Indeed, Alabama, North Carolina and Wisconsin began sending mail ballots to voters over the past week while the new system was still not active.
The Trump administration plan would require states to adopt a uniform envelope style and submit lists of eligible voters to an online portal. The Postal Service could refuse to deliver ballots to states that didn't comply.
A whistleblower report, though, said the postal system's requirements could lead to millions of mail ballots never being sent, because the portal wasn't properly built and a single bar code error could result in an entire batch of ballots being tossed out.
Democratic state officials and voting rights groups challenged the restrictions in court, arguing that the president had no constitutional authority to set election rules that would "virtually eliminate mail voting on the eve of a major election".
Lower courts agreed and blocked Trump's plan, including a preliminary injunction issued by a judge nominated by the president.
Trump has long opposed mail voting and falsely blamed it for his 2020 election loss to Democrat Joe Biden, even though he often uses that method to cast his own ballot, including as recently as this year.
with Reuters
