Three Democratic senators are filing a lawsuit challenging the constitutionality of President Donald Trump’s appointment of Matthew Whitaker to serve as acting attorney general, in the latest move by lawmakers to protect the probe being led by special counsel Robert Mueller III.

Sens. Richard Blumenthal of Connecticut, Sheldon Whitehouse of Rhode Island and Mazie Hirono of Hawaii filed a complaint Monday in the U.S. District Court for the District of Columbia. The complaint asks the court to declare Trump’s appointment of Whitaker as unconstitutional and to block him from his current role.

“Installing Matthew Whitaker so flagrantly defies constitutional law that any viewer of School House Rock would recognize it,” Blumenthal said in a statement. “Americans prize a system of checks and balances, which President Trump’s dictatorial appointment betrays.”

In a joint release announcing the move, the senators said they are being represented in the case by the nonprofit law firms Protect Democracy and the Constitutional Accountability Center.

Trump appointed Whitaker earlier this month after forcing the resignation of Jeff Sessions as attorney general, setting off a firestorm of controversy. Some Democrats have called for Whitaker to recuse himself from overseeing the special counsel investigation into Russian interference in the 2016 campaign due to his statements critical of the probe. Others, including the trio of senators who filed Monday’s lawsuit, have argued that Whitaker’s appointment violates the Constitution because Whitaker, who worked as Sessions’s chief of staff, is not a Senate-confirmed official.

Whitaker’s appointment also faces a lawsuit filed by Maryland Attorney General Brian Frosh, D, as well as a Supreme Court filing by lawyer Thomas C. Goldstein.

Amid the mounting legal challenges, the Justice Department’s Office of Legal Counsel released a 20-page memo last week defending the legality of the move, which it said was in line with similar appointments made by presidents George W. Bush and Barack Obama. The OLC memo also states that the appointment was authorized by the 1998 Federal Vacancies Reform Act. Critics contend that other statutes as well as the Constitution should take precedence over that measure.

The Department of Justice responded to Goldstein’s Supreme Court filing in a statement on Friday in which it maintained that Trump’s appointment of Whitaker is lawful and that it “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,” the DOJ said in the statement. “To suggest otherwise is to ignore centuries of practice and precedent.”