WASHINGTON (Reuters) -Donald Trump's administration asked the U.S. Supreme Court on Wednesday to let him remove Democratic members from two federal labor boards while legal fights over the Republican president's dismissals of them play out.

The Justice Department made the request after two separate Washington-based federal judges blocked Trump's firing of Cathy Harris from the Merit Systems Protection Board and Gwynne Wilcox from the National Labor Relations Board before their terms expire.

U.S. District Judges Rudolph Contreras and Beryl Howell separately upheld federal laws protecting officials serving in these posts from being fired without cause, rejecting Trump's argument that the measures passed by Congress encroach on authority granted to the president under the U.S. Constitution.

The U.S. Court of Appeals for the District of Columbia Circuit on Monday declined to pause the rulings by the judges while the cases proceed after an earlier ruling by that court had permitted the removals.

Trump's move to oust Harris and Wilcox was part of his far-reaching shakeup and downsizing of the U.S. government, including firing thousands of workers, dismantling federal agencies, installing loyalists in key jobs and purging career officials.

The legal fight over these firings has emerged as a key test of Trump's efforts to bring federal agencies meant by Congress to be independent from the president under his control.

Harris was appointed to the merit board in 2022 by Trump's Democratic predecessor Joe Biden to serve a seven-year term. Trump moved to fire her on February 10 after naming Henry Kerner, a Republican, as acting chair of the board.

Federal workers who lose their jobs can bring a challenge before the merit board, an independent three-member panel with quasi-judicial powers, seeking to be reinstated. More than 8,000 workers have filed cases with the board since Trump returned to the presidency in January, a massive surge for the agency.

The board has proved to be a potential roadblock to the Trump administration's efforts to carry out mass firings of probationary - meaning recently given their positions - workers. It has ordered the U.S. Department of Agriculture to temporarily reinstate thousands of workers who lost their jobs as part of Trump's layoffs.

Trump's efforts to remove Harris have threatened to leave the board without a two-seat quorum - making it unable to decide cases - after the term of Democratic member Raymond Limon expired on February 28.

In ruling in favor of Harris, Contreras said the statutory protections for board members from being removed without cause conform with the Constitution in light of a 1935 Supreme Court precedent in a case called Humphrey's Executor v. United States. In that case, the court ruled that a president lacks unfettered power to remove commissioners of the U.S. Federal Trade Commission, faulting then-President Franklin Roosevelt's firing of an FTC commissioner for policy differences.

Federal law permits a president to remove an official serving in this post only with cause such as inefficiency, neglect of duty or malfeasance.

Howell, the judge overseeing Wilcox's case, ruled on similar grounds to uphold almost identical job protections for the National Labor Relations Board member.

The NLRB, which has five members when fully stocked, enforces laws protecting the rights of private-sector workers to organize, join labor unions and advocate for better working conditions, and it oversees union elections. Federal labor law generally does not allow workers to sue for violations in court, so the board is often their only recourse.

Wilcox, the first Black woman to serve on the agency's board, was appointed to a second five-year term in 2023 by Biden for a new term. Trump moved to fire her on January 27.

The board reviews rulings by in-house judges in cases brought by the general counsel. Until it does, those orders cannot be enforced.

Hundreds of cases are pending before the board, including ones involving Amazon.com, Tesla, Walmart, Apple and dozens against Starbucks as it faces a nationwide unionization campaign.

Wilcox's would-be removal threatens to leave the board without a three-seat quorum because the board already had two vacancies. The U.S. Senate in December rejected a Democratic board member who had been nominated by Biden for a new term. Trump began his second term as president in January.