On Monday, three Democratic U.S. Senators filed a lawsuit challenging the legality of President Donald Trump‘s appointment of Matt Whitaker as acting attorney general.

Sens. Richard Blumenthal of Connecticut, Sheldon Whitehouse of Rhode Island and Mazie Hirono of Hawaii filed the suit, and are reportedly being represented by two organizations: the Constitutional Accountability Center and Protect Democracy.

Trump named Whitaker to lead the Justice Department after Jeff Sessions resigned on Nov. 7. Whitaker, a former U.S. Attorney in Iowa, is the first acting attorney general in the department’s history who hadn’t been working in a Senate-confirmed role at the time of his appointment. Many prominent Democratic lawmakers have stressed that Whitaker’s appointment to the position of attorney general is unconstitutional unless he is confirmed by the Senate.

Whitaker has already generated controversy because of his previous work at a company that defrauded clients and for heavily defending Trump against special counsel Robert Mueller, to the point of saying he believes he is legally permitted to undermine Mueller’s Russia investigation. Democrats have thus called for Whitaker’s recusal from the inquiry.

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Trump reportedly used the Federal Vacancies Reform Act and the Appointments Clause of the U.S. Constitution to name Whitaker as attorney general.

“There are over 160 instances in American history in which non-Senate confirmed persons performed, on a temporary basis, the duties of a Senate-confirmed position,” said Justice Department spokeswoman Kerri Kupec. “To suggest otherwise is to ignore centuries of practice and precedent.”

In an interview on Fox News on Sunday, Trump said he would not stop Whitaker should he actively try to limit Mueller’s probe.

“I would not get involved. It’s going to be up to him,” Trump said of Whitaker. “He’s going to do what’s right.”

“He’s a very smart person,” the president added of Whitaker, who once agreed with Trump that the Russia probe could be considered a “witch hunt.” 

Trump recently said he would soon submit his written answers to Mueller’s questions about Russian interference in 2016.