(Reuters) -Two Democrats on the U.S. Federal Trade Commission who were fired by President Donald Trump in March will urge a federal judge in Washington to declare the move illegal on Tuesday, in the latest showdown over the limits of presidential power.

Alvaro Bedoya and Rebecca Kelly Slaughter seek an order declaring their terminations unlawful and allowing them to resume their work at the agency, which enforces consumer protection and antitrust law.

The case is one of several testing a 90-year-old Supreme Court precedent that shields independent agencies from direct White House control. A ruling overturning it could reverberate far and wide, shaking the independence of agencies that regulate road safety, stock markets, telecommunications and monetary policy.

Bedoya and Slaughter say their terminations on March 18 openly defied a law allowing the president to fire FTC commissioners only for good cause, such as neglecting their duties.

The Supreme Court upheld that law in the 1935 case Humphrey's Executor v. U.S., after the last time a U.S. president attempted to fire an FTC commissioner over a policy disagreement.

Congress has the power to create agencies that serve legislative or judicial functions, and allowing the president to control those agencies violates the separation of powers, the Supreme Court ruled.

The Trump administration has argued Humphrey's Executor does not apply to the current FTC, which gained the authority to sue in federal court to block mergers and seek financial penalties after the case was decided.

As it now exists, the FTC should be considered part of the executive branch controlled by the president, not Congress, the administration has said.

Multiple courts have considered that argument and rejected it, saying the Supreme Court settled the matter, Slaughter and Bedoya said.

The FTC, currently led by three Republicans, is structured so that no more than three of its five commissioners come from the same party.

The case is playing out at the same time as similar challenges by members of the Merit Systems Protection Board and National Labor Relations Board who were fired by Trump.

The Supreme Court could rule at any time on whether the Trump administration must reinstate the NLRB and MSPB members, while this case is being reviewed.