WASHINGTON (Reuters) - Millions of U.S. government workers face a Thursday deadline to tell the Trump administration whether they will accept a buyout offer or remain in their jobs, but still face the possibility of being thrown out of work if they decide to stay.

President Donald Trump's aggressive drive to reduce the size of the government in a matter of weeks is unprecedented. It has caused turmoil in the U.S. capital, and sparked protests and union warnings that it may violate multiple laws.

More than 40,000 federal employees had accepted the buyout offer as of Wednesday night, according to a source.

It is not clear how many of the federal government's 2.3 million civilian employees will ultimately accept the offer, which promises to pay their salaries until October if they resign by Thursday.

Others are fighting the proposal in court.

Unions representing federal workers sued to block the "deferred resignation program" last week. A federal judge in Boston is to consider their request at 1 p.m. Eastern time (1800 GMT) on Thursday.

Those who don't accept the offer have been told their jobs could be at risk as the administration moves ahead with its overhaul of government operations.

The buyout encompasses not only employees at domestic agencies like the Department of Labor that have traditionally drawn the ire of Trump's Republicans, but intelligence agencies like the CIA as well.

The buyout is part of a government overhaul, which is being led by Trump's billionaire adviser Elon Musk. It has resulted in purges of personnel in a number of departments and has significantly scaled down America's main humanitarian aid agency, the U.S. Agency for International Development.

More cuts are likely. One source at the Office of Personnel Management said officials upped their demand for layoffs on Wednesday from 30% to 70% of all staff.

Trump's lieutenants at the Justice Department have already fired prosecutors who worked on cases involving those who attacked the U.S. Capitol on January 6, 2021, and have demanded the names of up to 6,000 FBI agents who were involved.

The White House says it is following through on Trump's campaign promise to cut wasteful spending and slim down a bureaucracy that many conservatives see as left-leaning and unresponsive to the president's agenda.

Democrats say the effort is illegal, as the U.S. Constitution gives Congress authority over spending matters.

"Everyone is stressed and frustrated. The mood is awful," said one worker at the Department of Health and Human Services, who spoke on condition of anonymity.

Republican lawmakers, who control both chambers of Congress, have cheered the effort, and Democrats have been unable to stop it.

All 47 Democrats in the U.S. Senate are expected on Thursday to vote against confirming Trump's pick for White House budget chief, Russell Vought, but he is still expected to win confirmation with support from the chamber's 53 Republicans.