WASHINGTON (Reuters) -The U.S. Supreme Court allowed Donald Trump on Thursday to keep two Democratic members of federal labor boards away from their posts while their challenge to his firing of them proceeds in a legal dispute that tests the Republican president's power over independent government agencies.

The court temporarily blocked orders by two separate Washington-based federal judges that had shielded Cathy Harris from being dismissed from the Merit Systems Protection Board and Gwynne Wilcox from being removed from the National Labor Relations Board before their terms expire. Their legal challenges are ongoing in lower courts.

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

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.

Chief Justice John Roberts on April 9 temporarily halted orders by the two judges who had blocked Trump's firing of Harris and Wilcox, giving the justices more time to decide how to proceed. The labor boards after that decision by Roberts had confirmed that Harris and Wilcox were no longer in their posts.

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 April 7 declined to pause the rulings by the judges while the cases proceed after an earlier ruling by that court had permitted the removals.

Justice Department lawyers in a Supreme Court filing on April 9 said the lower court rulings had created an untenable situation.

"The president should not be forced to delegate his executive power to agency heads who are demonstrably at odds with the administration's policy objectives for a single day - much less for the months that it would likely take for the courts to resolve this litigation," they wrote.

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. The board has proven to be a potential roadblock to the Trump administration's efforts to carry out mass firings of probationary workers, meaning those recently given their positions.

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 a commissioner on that agency 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.

Justice Department lawyers in filings to the Supreme Court contended that the judges read the Humphrey's Executor precedent too broadly. The lawyers argued that the precedent upheld tenure protections for Federal Trade Commission members because that agency does not significantly encroach on presidential authority, while the Merit Systems Protection Board and National Labor Relations Board, by contrast, "wield substantial executive power."

The National Labor Relations Board, 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 National Labor Relations 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.

Without Wilcox, the board would lack its needed three-seat quorum because it already had two vacancies.