The US Supreme Court is set to take on US President Donald Trump's refusal to turn over his tax returns and financial records to Congress and New York prosecutors.
The court may also use the occasion to better define the limits of presidential immunity.
Its nine justices, confined at home by the coronavirus pandemic, will question lawyers for both sides by phone in a highly anticipated session to be broadcast live.
The hearing, initially set for late March, is being held now to allow time for the justices to render a decision before the presidential election in November, when Mr Trump seeks a second term.
The former property magnate, who used his fortune as a prop for his 2016 election campaign, is the first president since Richard Nixon in the 1970s to refuse to release his tax returns.
It has prompted speculation about his true worth and possible financial entanglements.
"There is clearly something in these documents that the president does not want us to see," Steven Mazie, an author and educator, said during an online seminar.
Since retaking control of the House of Representatives in midterm elections in 2018, the Democratic opposition has been eager to find out what that "something" might be.
Several congressional committees have issued subpoenas to Mr Trump's longtime accounting firm, Mazars, and to lenders Deutsche Bank and Capital One, demanding his financial records from 2011 to 2018.
Manhattan prosecutor Cyrus Vance, a Democrat, made a similar demand to Mazars as part of an investigation into payments to the adult actress known as Stormy Daniels to buy her silence about an alleged liaison with the billionaire.
Mr Trump sued to block the documents' release.
"What they are doing is not legal," he said on Twitter. "The witch hunt continues."
Having lost his argument in the lower courts, Mr Trump turned to the nation's highest legal body.
With two of his conservative appointments on the nine-justice bench, the high court has taken a clear turn to the right.
They will devote the first hour of Tuesday's oral arguments to the congressional subpoenas, highlighting a fierce battle over the legislature's investigative powers.
"Unleashing each and every House committee to torment the president with legislative subpoena after legislative subpoena is a recipe for constitutional crisis," Mr Trump's lawyers said in a brief to the court.
But such requests are nothing new, House lawyers responded in their own brief, giving examples involving presidents Richard Nixon, a Republican, and Jimmy Carter, a Democrat.
"What is unprecedented is the extraordinary breadth of the arguments that President Trump and the solicitor general make about the supposed power of a president to thwart investigations," they said.
The Supreme Court may be tempted to sidestep the central issue.
In late April, it asked both sides to respond in writing to the question of whether the matter was political and not legal in nature. If it is, the justices can close the file without taking a position.
Both sides on Friday denied it was political, clearly hoping for resolution.
In a second phase of Tuesday's session, the justices will take up the case involving the Manhattan prosecutor, which raises the question of presidential immunity before the law.
Mr Trump's lawyers say a president enjoys total immunity for as long as he is in the White House.
One lawyer even argued before an appeals court that the president could shoot someone dead on New York's Fifth Avenue and face no legal penalty – while in office.
"Nothing could be done?" a sceptical judge asked.
"That is correct," Mr Trump's lawyer replied.
To his legal team, the need for immunity is "particularly acute when it comes to state and local prosecutors".
"The president must be allowed to execute his official functions without fear that a state or locality will use criminal process to register their dissatisfaction with his performance," they wrote in their brief.
But law professors Claire Finkelstein and Richard Painter said this was contradicted by the record of the Supreme Court.
It required Nixon to turn over secret White House recordings to the special prosecutor investigating the Watergate scandal.
The two professors said in a brief that an expansive interpretation of presidential immunity poses a "grave threat to the rule of law".
If the Supreme Court accepted the Trump team's arguments, they said: "It will fundamentally alter the basic principles of accountability on which our democracy depends."
