WASHINGTON (AFP) -- Donald Trump on Thursday asked the Supreme Court to bar a prosecutor from obtaining his tax returns, arguing that as president he has blanket immunity.

Trump, a former New York businessman, is the first American president since Richard Nixon not to make his tax returns public, arguing they're under audit by the Internal Revenue Service.

Opposition Democrats have turned to the courts to force their release in a case that's considered a crucial test of the separation of powers. And if the US high court -- where conservative judges hold the majority -- decides to take up the case, their decision will receive endless scrutiny.

Manhattan District Attorney Cyrus Vance Jr, a Democrat, has demanded Trump's tax returns dating back to 2011 as part of an investigation into payments made by Michael Cohen, the president's former personal attorney, to Stormy Daniels, an adult film actress who claimed to have had a sexual liaison with Trump before he ran for president in 2016.

Last week, a federal appeals court ruled that Trump must hand over the documents.

But the president's attorneys argue that he enjoys immunity both from prosecution and from any acts of investigation.

"For the first time in our nation's history, a state or local prosecutor has launched a criminal investigation of the President of the United States and subjected him to coercive criminal process," Trump's attorney Jay Sekulow said, according to US media.

"Politically motivated subpoenas like this one are a perfect illustration of why a sitting president should be categorically immune from state criminal process," he said.

At a hearing on the case on October 23, one of Trump's lawyers went so far as to argue that the president would have immunity even if he shot someone in the street -- a statement immediately challenged by jurists and mocked by Democrats.

There have been various other attempts to obtain the returns by Democratic lawmakers in New York and in the House of Representatives. (AFP)