Three sitting United States senators have filed a lawsuit challenging the legitimacy of President Donald Trump’s appointment of Acting Attorney General Matt Whitaker following the forced resignation of former Attorney General Jeff Sessions.

Sens. Richard Blumenthal (D-Conn.), Mazie K. Hirono (D-Hawaii) and Sheldon Whitehouse (D-R.I.), who filed the lawsuit, are being represented by the groups Protect Democracy and the Constitutional Accountability Center. 

Whitaker, who Trump appointed to the role after Sessions resigned 12 days ago, is the first acting attorney general in Justice Department history who hadn’t been serving in a Senate-confirmed position at the time of his appointment. The Justice Department’s Office of Legal Counsel issued a 20-page opinion last week arguing that Whitaker’s appointment was legitimate, while conceding that the last time that the acting attorney general wasn’t Senate-confirmed was in 1866, before the creation of the DOJ.

Justice Department spokeswoman Kerri Kupec, responding to another lawsuit challenging Whitaker’s appointment that was filed on Friday, said in a statement on Monday that Whitaker’s appointment “is lawful and comports with the Federal Vacancies Reform Act, the Appointments Clause of the U.S. Constitution, Supreme Court precedent, past Department of Justice opinions, and actions of U.S. Presidents, both Republican and Democrat.”

“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,” Kupec said. “To suggest otherwise is to ignore centuries of practice and precedent.