Three Democratic senators Monday sued to challenge President Donald Trump’s appointment of Matthew Whitaker as acting attorney general.

Amid concerns Whitaker was chosen to protect Trump from the Russia collusion probe of Special Prosecutor Robert Mueller, the senators said the president violated the constitution by appointing him to lead the Department of Justice without first seeking Senate confirmation.

“The US Senate has not consented to Mr Whitaker serving in any office within the federal government, let alone the highest office of the DOJ,” they told the federal court in Washington.

“The constitutional requirement that principal federal officers be appointed only with the Senate’s ‘advice and consent’… was adopted by our nation’s founders as an important check on the power of the president,” they said.

One of the three senators, Sheldon Whitehouse, accused Trump of installing his “lackey” to undermine the Mueller probe.

– Whitaker replaced Jeff Sessions –

Trump named the former prosecutor and television commentator to be acting attorney general on November 7 after forcing attorney general Jeff Sessions to resign.

Many critics believe Whitaker was chosen to insulate Trump as Mueller’s investigation gets nearer to the White House. Last year he repeatedly attacked the investigation as too broad and legally questionable and called to defund it.

Sessions had recused himself from overseeing the Russia probe, handing the responsibility to Deputy Attorney general Rod Rosenstein.

Normally Rosenstein, who has gone through Senate confirmation, would have been made acting attorney general until a new nominee could be vetted.

But now Rosenstein has had to step aside from overseeing Mueller, leaving that power to Whitaker.

The White House and the Justice Department both argue that the appointment was legal.

In a statement released on Monday, Justice Department spokeswoman, Kerri Kupec, said it was based on “centuries of practice and precedent” and “comports with… actions of US presidents, both Republican and Democrat.”

In an interview with Fox News on Sunday, Trump defended Whitaker, but avoided addressing his comments on the Mueller investigation.

“He’s a very smart person. A very respected person. He’s going to do what’s right,” the president said.