WASHINGTON (Reuters) - U.S. President Donald Trump threatened on Wednesday to shut down Congress so he could fill vacancies in his administration without Senate confirmation, saying he was frustrated lawmakers were not in Washington to vote on his nominees for federal judgeships and other government positions.

“The current practice of leaving town, while conducting phony pro forma sessions, is a dereliction of duty that the American people cannot afford during this crisis,” an angry Trump told reporters at his daily White House briefing on the coronavirus crisis.

“It is a scam that they do. It’s a scam and everyone knows it, and it’s been that way for a long time,” Trump said.

No U.S. president has ever used the authority, included in the Constitution, to adjourn both chambers of Congress if they cannot agree on a date to adjourn.

It was not immediately clear if Congress’ current absence from Washington because of the global pandemic could be classified as being due to a failure to agree on an adjournment date.

The Senate and House of Representatives have both announced plans to return to Washington on May 4, and had been scheduled to be out of Washington for two weeks in April for their annual Easter break even before the coronavirus crisis.

Senate Majority Leader Mitch McConnell discussed nominations with Trump on Wednesday and promised to find ways to confirm those “considered mission-critical” to the pandemic, a McConnell spokesman said.

“However, under Senate rules, that would take consent from Democratic leader Chuck Schumer,” the spokesman said.

Members of the Republican-controlled Senate and Democratic-led House have been out of Washington since mid-March, as government officials and health experts recommended that Americans stay home to avoid spreading the deadly coronavirus.

As is typical during congressional recesses, the House and Senate have held regular “pro forma” sessions, brief meetings that can last less than a minute, rather than formally adjourning.

As long as those short sessions are held regularly, the president cannot make “recess appointments.

After lamenting that some nominees have waited months to be confirmed, Trump threatened to force Congress to adjourn. He acknowledged that such a move would face a legal challenge, but was confident he would prevail.

“We’ll probably be challenged in court, and we’ll see who wins,” Trump said.

Any legal challenge would take many months.

Trump’s appointment of conservative federal judges is an important part of his appeal to Republican voters as he runs for re-election in November. His campaign issued a tweet about his adjournment threat almost immediately after he made it.

Trump said his nominees would help deal with the pandemic, without explaining how that would be the case.