The US Supreme Court heard arguments on Thursday on whether a former president is immune from criminal prosecution and appeared poised to issue a ruling that could further delay Donald Trump's trial on charges of conspiring to overturn the 2020 election.

The historic case -- the last in the court's current term – has far-reaching implications for executive power and Trump's multiple legal issues as he seeks the White House again.

"We're writing a rule for the ages," said Justice Neil Gorsuch, one of three conservative justices appointed to the nation's highest court by the former Republican president.

At least four, and possibly five, of the conservative justices on the nine-member court appeared to take issue with a lower court ruling that a former president does not enjoy "absolute immunity" from criminal prosecution after leaving office.

Chief Justice John Roberts, a conservative, told Michael Dreeben, representing Special Counsel Jack Smith, who brought the election conspiracy charges against Trump, that he had "concerns" with the lower court ruling.

"As I read it, it says simply, 'a former president can be prosecuted because he's being prosecuted,'" Roberts said.

"Why shouldn't we either send (the case) back to the Court of Appeals or issue an opinion making clear that that's not the law?"

Sending the case back to the lower court for further review would almost certainly delay Trump's election conspiracy trial until after the November vote, when he is expected to face off once again against Democrat Joe Biden.

Justice Samuel Alito, another conservative, asked why – without immunity – a president won't just "pardon themselves from anything that they might have been conceivably charged with committing?"

John Sauer, representing Trump, told the court that "without presidential immunity from criminal prosecution there can be no presidency as we know it."

"Every current president will face de facto blackmail and extortion by his political rivals while he is still in office," Sauer said. (AFP)