The United States Supreme Court on Monday appeared poised to hand President Donald Trump a significant boost in executive power after conservative justices signaled that they would uphold the dismissal of a member of the Federal Trade Commission (FTC).

President Trump fired Democratic FTC member Rebecca Slaughter in March, four years before the end of her term and in contradiction of a 90-year-old legal precedent which affords tenure protection to the heads of independent agencies.

A lower court had initially ruled that Trump had exceeded his presidential authority by dismissing Slaughter but, during a two-hour appeal hearing, the Justice Department argued that tenure protections unlawfully encroached on presidential power.

And the argument appeared on Monday to have earned sympathy from conservative justices, who have a 6-3 Supreme Court majority and who have called for the precedent in question — known as "Humphrey's Executor v. United States" — to be overturned.

Conservatives argue precedent is out of date

"Humphrey's Executor must be overruled," said US Solicitor General D. John Sauer, arguing for the Trump administration, arguing that the 1935 precedent has "not withstood the test of time."

Conservative Chief Justice John G. Roberts appeared to concur, suggesting that the ruling, which rebuffed an attempt by President Franklin Roosevelt to fire an FTC member over policy differences, was a relic of the past which had applied to an FTC which was far less powerful in the 1930s than it is today.

"Humphrey's Executor is just a dried husk of whatever people used to think it was," Roberts said. "It was addressing an agency that had very little, if any, executive power and that may be why they were able to attract such a broad support on the court at the time."

Solicitor Sauer said the existence of the precedent "continues to tempt Congress to erect, at the heart of our government, a headless fourth branch insulated from political accountability and democratic control."

Liberals warn of boost in presidential power

Liberal justices, however, warned that overturning the precedent and upholding Slaughter's dismissal would signal a massive increase in presidential power for Trump.

"The result of what you want is that the president is going to have massive, unchecked, uncontrolled power – not only to do traditional execution, but to make law through legislative and adjudicative frameworks," argued Justice Elena Kagan, adding:

"What you are left with is a president ... with control over everything, including over much of the lawmaking that happens in this country."

Her liberal colleague Justice Sonia Sotomayor said that independent agencies such as the FTC have existed throughout US history, and for good reason.

"Neither the King, nor Parliament nor prime ministers in England at the time of the founding [of the United States] ever had an unqualified removal power," Sotomayor argued, adding:

"You're asking us to destroy the structure of government and to take away from Congress its ability to protect its idea that a government is better structured with some agencies that are independent."

'Real-world realities'

Given Trump's recent testing of the constitutional limits of presidential powers in areas including immigration, tariffs and domestic military deployments, Kagan called on the court not to ignore "the real-world realities" of what its decisions can do.

"We are asking the court to adhere to all of its precedents and to give effect to the collective wisdom and experience of all three branches of government," added Amit Agarwa, a lawyer representing Slaughter.

By contrast, he continued, the administration is "asking you to abandon precedent after precedent, after precedent."

Edited by: Jenipher Camino Gonzalez