The US Supreme Court has denied President Donald Trump's attempt to restrict mail-in voting ahead of the November midterm elections.

Trump had requested that the top court lift a lower court's order which blocked his bid to restrict the mail-in voting.

In a concurring opinion with the majority, conservative Justice Brett Kavanaugh said that while the Republican president's plan might ultimately prove legal, "state and local election officials do not have sufficient time to reasonably implement the rule before the elections."

Conservative Justices Samuel Alito and Clarence Thomas publicly dissented from the brief order.

The Supreme Court's decision means states are allowed to continue sending out mail ballots under the same processes they have used for years.

The case has major implications because nearly one-third of the country votes by mail.

What do Trump's mail-in restrictions entail?

Trump's plan would obligate states to adopt a uniform envelope style and provide lists of mail ballot recipients to the Postal Service, which could then refuse to send out ballots that fail to comply with the new standards.

A whistleblower report has alleged that the postal system's requirements could result in millions of mail ballots never getting sent as the portal was not properly built and could therefore experience glitches.

Critics argue that the restrictions could disrupt the delivery of thousands of legitimate ballots ahead of the November 3 vote, for which states including Alabama, North Carolina and Wisconsin have already begun sending mail ballots to voters over the past week.

Democratic state officials and voting rights groups challenged the restrictions in court, arguing that the president has no constitutional authority to set election rules that would "virtually eliminate mail voting on the eve of a major election."

Lower courts have pulled the brakes on Trump's new system, which is why he had resorted to the Supreme Court.

Why is Trump trying to restrict mail-in ballots?

In March, Trump signed an executive order to restrict mail-in voting, saying it is vulnerable to fraud, without providing evidence.

The US president, who himself often relies on mail-in voting to cast his own ballot, has long opposed the system. He even falsely blamed it for his 2020 election loss to Democrat Joe Biden.

Restricting mail-in ballots would benefit Republicans, various surveys have concluded, as Democratic voters disproportionately use the system to cast their ballots.

Polls predict that the Republican Party is in serious danger of losing its narrow hold of Congress in November, particularly the House of Representatives.

What are the reactions to the Supreme Court decision?

Democrat California Governor Gavin Newsom, whose state led one of the lawsuits against the restrictions, welcomed the Supreme Court's decision.

"Today is a good day for democracy, the rule of law and the American people looking to exercise their constitutional right to vote," he said. "Trump's attacks on democracy these last 20 months have been nothing short of un-American."

Also welcoming the ruling was Sophia Lin Lakin, voting rights project director at the American Civil Liberties Union, which helped to represent some of the challengers to the mail-in voting rule.

"This administration keeps testing the limits of its power, as if the rules governing our ​elections are merely suggestions to ignore when they don't suit the president's agenda. But the Constitution gives states and Congress the power to set the rules for federal elections — not the president, and not the Postal Service," she said.

Senate Democratic Leader Chuck Schumer described the ruling as "one of the most important cases to protect voting rights before the Supreme Court."

"Donald Trump: you lost another one. The President’s attempt to screw around with vote by mail was obscene. Finally, his scheme is dead."

Edited by: Wesley Dockery

If you rely on our team for trusted reporting, please take a moment to select us as your Preferred Source on Google by clicking here and hitting the "star" or "preferred" button, so you'll always see our verified news first.