U.S. President Donald Trump on Thursday asked the Supreme Court to reverse a lower court ruling that directed his longtime accounting firm to hand over eight years of his tax returns to New York prosecutors, in what amounts to a major test of his ability to avoid investigation while in the White House. 

What's Trump legal argument?

Trump appealed a November 4 ruling by the New York-based 2nd U.S. Circuit Court of Appeals that prosecutors can enforce a subpoena demanding his personal and corporate tax returns from 2011 to 2018 from accounting firm Mazars LLP.

"In our petition, we assert that the subpoena violates the U.S. Constitution and therefore is unenforceable. We are hopeful that the Supreme Court will grant review in this significant constitutional case and reverse the dangerous and damaging decision of the appeals court," said Jay Sekulow, one of Trump's lawyers.

The legal questions include whether the subpoena violates the part of the U.S. Constitution that lays out the power of the president. All other modern day presidents have released their tax returns. 

Who requested the tax returns?

The office of Manhattan District Attorney Cyrus Vance, a Democrat, is seeking the returns as part of a criminal investigation into Trump and the Trump Organization, the president's family real estate business.

Trump's lawyers have said he cannot be subjected to any criminal process while he remains president, a broad interpretation of presidential immunity. Even if he is not immune, the subpoena is not valid because Vance has not shown any specific need for the information, Trump's lawyers argued.

"There has been broad bipartisan agreement, for decades if not for centuries, that a sitting president cannot be subjected to criminal proceedings," Trump's lawyers wrote.

Vance's investigation involves alleged hush money payments to two women prior to the 2016 election who said they had sexual relationships with Trump, which he denies.

What happens next?

If the justices decline to hear Trump's appeal, the lower court ruling would stand, clearing the way for Vance to obtain the documents.

If they opt to take up the appeal, the justices then must decide whether to hear the case in their current term, which ends in June, or in their next term that begins in October 2020, likely pushing any decision until after the November 2020 presidential election.

Vance agreed not to seek enforcement of the subpoena while Trump appealed the matter on an expedited schedule. 

In a separate case in which the president has unsuccessfully fought efforts by House Democrats to obtain his financial records from Mazars, Trump's lawyers are due to file an emergency application at the Supreme Court on Friday, Sekulow said.

Mazars has said it will comply with the subpoenas, if the courts direct it to do so. 

The court's 5-4 conservative majority includes two justices Trump appointed: Brett Kavanaugh and Neil Gorsuch. Trump has prevailed at the Supreme Court on major issues such as his travel ban on people entering the United States from several Muslim-majority countries, but some legal experts have predicted that he may not fare as well on cases focusing on his personal conduct.

(With input from Reuters)