Connecticut Democratic Sen. Richard Blumenthal has filed a lawsuit with fellow Democratic senators challenging the appointment of Matthew Whitaker as acting U.S. attorney general, arguing he is ineligible to lead the Justice Department in an interim capacity because he did not rise from a role that required Senate confirmation.

The lawsuit—filed by Blumenthal, Sheldon Whitehouse and Mazie Hirono—seeks a court order preventing Whitaker from continuing to serve as the acting attorney general. The complaint, joining other legal challenges, is the first that squarely addresses Whitaker’s appointment.

Whitaker ascended to acting U.S. attorney general following the forced resignation of former U.S. Attorney General Jeff Sessions after the midterms. Whitaker, who served under the George W. Bush administration as a U.S. attorney in Iowa, had been Sessions’s chief of staff since late last year.

“Installing Matthew Whitaker so flagrantly defies constitutional law that any viewer of School House Rock would recognize it,” Blumenthal said. “President Trump is denying Senators our constitutional obligation and opportunity to do our job: scrutinizing the nomination of our nation’s top law enforcement official. The reason is simple: Whitaker would never pass the advice and consent test. In selecting a so-called ‘constitutional nobody’ and thwarting every Senator’s constitutional duty, Trump leaves us no choice but to seek recourse through the courts.”

“Unlawfully denying legislators their right to cast an effective vote robs them of one of their core powers and responsibilities,” lawyers at the Constitutional Accountability Center and the Protect Democracy Project, representing the three senators, said in their complaint in U.S. District Court for the District of Columbia.

Several other cases have raised challenges to Whitaker’s appointment, including court papers filed Friday in the U.S. Supreme Court. The justices were urged in a Second Amendment case to confront the lawfulness of Whitaker’s appointment.

The Justice Department defended Whitaker’s appointment Monday in a statement:

“President Trump’s designation of Matt Whitaker as Acting Attorney General of the United States 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. To suggest otherwise is to ignore centuries of practice and precedent.”

In the new lawsuit, the three Democrats—all members of the Senate Judiciary Committee—said Whitaker was ineligible to step in as the interim attorney general because his past role as a Sessions aide did not require Senate confirmation.

The lawsuit highlighted Whitaker’s past involvement as an adviser to a company that shut down after being accused by the Federal Trade Commission of bilking consumers out of $26 million. Critics also contend Whitaker is predisposed, based on media appearances and written comments, to curtail Special Counsel Robert Mueller III’s ongoing investigation of Donald Trump’s campaign ties to Russia.

“The U.S. Senate has not consented to Mr. Whitaker serving in any office within the federal government, let alone the highest office of the DOJ,” Monday’s complaint said. “Indeed, before deciding whether to give their consent to Mr. Whitaker serving in such a role, plaintiffs and other members of the Senate would have the opportunity to consider his espoused legal views, his affiliation with a company that is under criminal investigation for defrauding consumers, and his public comments criticizing and proposing to curtail ongoing DOJ investigations that implicate the president.”

The lawsuit has ramped up the legal scrutiny surrounding Whitaker’s appointment and represents the highest-profile challenge to his ongoing leadership of the Justice Department. Earlier this month, a Texas businessman facing prosecution over the distribution of allegedly substandard pet food ingredients filed his own challenge within his criminal case to Whitaker’s service as acting attorney general.

In that challenge, lawyers for the former agricultural products executive Doug Haning said Whitaker’s leadership of the Justice Department was unlawful because he was not appointed “with the advice and consent of the Senate.” The lawsuit, first reported by Politico, argues that Whitaker holds his authority “improperly and in direct contravention of the Appointments Clause.”

“The stakes are too high to allow the president to install an unconfirmed lackey to lead the Department of Justice—a lackey whose stated purpose, apparently, is undermining a major investigation into the president,” said Whitehouse. Unless the courts intercede, this troubling move creates a plain road map for persistent and deliberate evasion by the executive branch of the Senate’s constitutionally mandated advice and consent. Indeed, this appointment appears planned to accomplish that goal,” said Whitehouse.

“Donald Trump cannot subvert the Constitution to protect himself and evade accountability,” Hirono added. “We want the court to make clear that the Senate must confirm Matthew Whitaker’s appointment as Acting Attorney General—otherwise this temporary appointment violates the Constitution’s Appointments Clause.”